Image display apparatus and image display methods

ABSTRACT

Provided are image display apparatus and image display methods capable of suitably making correction for variation of driving conditions due to an electric resistance of matrix wiring of a display panel by downsized hardware. The apparatus and methods involve a device of calculating voltage drop amounts caused by the resistance of row wires, for input image data, and a device of calculating image data with correction for the voltage drop amounts (corrected image data). An overflow processing circuit is provided so as to prevent overflow of the image data after the correction from an input range of a modulator, and the overflow is prevented by a gain. Since a gradation converter for changing a gradation conversion characteristic by a gain is provided in the stage preceding to the configuration for making the correction for influence of the voltage drop, it becomes feasible to cancel saturation characteristics of phosphors and to display images with high quality thereby.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to image display apparatus such astelevision receivers, display devices, etc. for receiving a televisionsignal or a display signal of a computer or the like to display animage, using a display panel equipped with a plurality of image formingdevices matrix-wired, and to image display methods.

[0003] 2. Related Background Art

[0004] The conventional apparatus was provided with (n×m) image formingdevices arrayed in a matrix pattern as wired to m row wires and n columnwires and was configured to implement sequential scanning of the rowwires and modulation in the column direction, thereby driving a devicegroup of each row simultaneously.

[0005] In the case of this driving, there occurred a voltage drop due tothe electrical resistance of wiring, in the row wiring, so as to pose aproblem of a defect due to decrease of the voltage placed between thetwo ends of the display devices.

[0006] In order to make correction for decrease of luminance due to thevoltage drop caused by the wiring resistance of the electricalconnection wires and others to the display devices as described above,Japanese Patent Application Laid-Open No. 08-248920 discloses thetechnology about the image display apparatus having a configuration ofcalculating correction data for the voltage drop by statisticaloperation and combining electron beam requirements with correctionvalues.

[0007] The configuration of this image display apparatus described inthe Japanese application is presented in FIG. 38. The configurationassociated with the correction for data in this apparatus is roughly asdescribed below. First, luminance data of one line in a digital imagesignal is added up at an adder 206 and a correction factor correspondingto the sum is read out of a memory 207. On the other hand, the digitalimage signal is subjected to serial-parallel conversion at a shiftregister 204, the resultant parallel signals are retained for apredetermined period of time at a latch 205, and then they are fed inpredetermining timing into multipliers 208 provided for the respectivecolumn wires. At each multiplier 208 the luminance data for each columnwire is multiplied by the correction data read out of the memory 207,the resultant corrected data is transferred to a modulation signalgenerator 209, modulation signals corresponding to the corrected dataare generated at the modulation signal generator 209, and an image isdisplayed on a display panel on the basis of the modulation signals.This apparatus is configured to perform statistical operation processinglike calculations of the sum and average for the digital image signal,e.g., the adding operation of luminance data of one line in the digitalimage signal at the adder 206, and make the correction based on thisvalue.

[0008] The conventional configuration as described above, however,required the large-scale hardware including the multipliers for therespective column wires, the memory for providing the output of thecorrection data, the adder for supplying an address signal to thememory, and so on.

[0009] An object of the present invention is to provide image displayapparatus and image display methods capable of making appropriatecorrection for a luminance variation and a chromaticity variation causedby a variation in driving conditions due to the electrical resistance ofmatrix wiring of the display panel by smaller-scale hardware.

SUMMARY OF THE INVENTION

[0010] In order to achieve the above object, an image display apparatusaccording to the present invention is an image display apparatuscomprising:

[0011] a plurality of image forming devices connected to a plurality ofrow wires and column wires respectively and arranged in a matrixpattern;

[0012] scanning means connected to the row wires;

[0013] modulating means connected to the column wires;

[0014] gradation converting means for converting a gradationcharacteristic of input image data;

[0015] corrected image data calculating means for calculating correctedimage data, which is image data after correction for influence of avoltage drop caused by a resistance of the row wires and scanning means,for an output of the gradation converting means;

[0016] the modulating means outputting modulation signals to the columnwires, with entry of the corrected image data,

[0017] wherein the gradation conversion characteristic is acharacteristic of making correction for a light emission characteristicof the image forming devices in an absent state of the voltage drop.

[0018] Another image display apparatus of the present invention is animage display apparatus comprising:

[0019] a plurality of image forming devices connected to a plurality ofrow wires and column wires respectively and arranged in a matrixpattern;

[0020] scanning means connected to the row wires;

[0021] modulating means connected to the column wires;

[0022] gradation converting means for converting a gradationcharacteristic of input image data;

[0023] corrected image data calculating means for calculating correctedimage data, which is image data after correction for influence of avoltage drop caused by a resistance of the row wires and scanning means,for an output of the gradation converting means; and

[0024] amplitude adjusting means having a function of multiplying databy a factor for adjustment of the amplitude of the corrected image dataso that the amplitude of the corrected image data matches an input rangeof the modulating means, wherein the gradation converting means has agradation conversion characteristic corresponding to the factor, and

[0025] wherein the modulating means outputs modulation signals to thecolumn wires, with entry of the corrected image data amplitude-adjustedby the amplitude adjusting means.

[0026] Still another image display apparatus of the present invention isan image display apparatus comprising:

[0027] a plurality of electron-emitting devices connected to a pluralityof row wires and column wires respectively and arranged in a matrixpattern;

[0028] scanning means connected to the row wires;

[0029] modulating means connected to the column wires;

[0030] gradation converting means for performing gradation conversion ofinput image data;

[0031] corrected image data calculating means for calculating correctedimage data, which is image data after correction for influence of avoltage drop caused by a resistance of the row wires and scanning means,for an output of the gradation converting means; and

[0032] amplitude adjusting means having a function of multiplying databy a factor for adjustment of the amplitude of the corrected image dataso that the amplitude of the corrected image data matches an input rangeof the modulating means,

[0033] in which the gradation converting means has a gradationconversion characteristic corresponding to the factor, and

[0034] in which the modulating means outputs modulation signals to thecolumn wires, with entry of the corrected image data amplitude-adjusted;

[0035] wherein with entry of nonzero, uniform image data common to allcolors, a pulse width of an output pulse from the modulating means closeto an output terminal of the scanning means becomes shorter than a pulsewidth of an output pulse from the modulating means far from the outputterminal of the scanning means, and

[0036] saturation characteristics of phosphors dependent upon emittedcharge amounts of the electron-emitting devices are further canceled, soas to implement such driving that any image data uniform and common toall the colors is displayed at almost equal color temperature of whitecolor, independent of emission luminance.

[0037] An image display method according to the present invention is animage display method by an image display apparatus comprising aplurality of electron-emitting devices connected to a plurality of rowwires and column wires one each respectively and arranged in a matrixpattern, scanning means connected to the row wires, modulating meansconnected to the column wires, and phosphors opposed to theelectron-emitting devices, the image display method comprising:

[0038] a step of calculating emitted charge amount requirements withcorrection for light emission characteristics of the phosphors againstemitted charge amounts, according to image data as luminancerequirements; and

[0039] a step of calculating corrected image data with correction forvariation of the emitted charge amounts due to influence of a voltagedrop caused by a resistance of the row wires and scanning means,according to the emitted charge amount requirements calculated,

[0040] wherein the modulating means applies pulse waveforms according tothe corrected image data thus calculated, to the column wires.

[0041] Another image display method of the present invention is an imagedisplay method by an image display apparatus comprising a plurality ofelectron-emitting devices connected to a plurality of row wires andcolumn wires one each respectively and arranged in a matrix pattern,scanning means connected to the row wires, and modulating meansconnected to the column wires, the image display method comprising:

[0042] a step of performing gradation conversion of canceling a lightemission characteristic of the electron-emitting devices in an absentstate of a voltage drop caused by a resistance of the row wires andscanning means, for input image data; and

[0043] a step of making correction for influence of the voltage dropcaused by the resistance of the row wires and scanning means, for anoutput in the step of performing the gradation conversion of cancelingthe light emission characteristic,

[0044] wherein the modulating means applies pulse waveforms to thecolumn wires according to an output in the step of making the correctionfor the influence of the voltage drop.

[0045] Still another image display method of the present invention is animage display method by an image display apparatus comprising aplurality of electron-emitting devices connected to a plurality of rowwires and column wires one each respectively and arranged in a matrixpattern, scanning means connected to the row wires, and modulating meansconnected to the column wires, the image display method comprising:

[0046] a step of converting a gradation characteristic of input imagedata;

[0047] a step of making correction for influence of a voltage dropcaused by a resistance of the row wires and scanning means, for anoutput in the step of converting the gradation characteristic,

[0048] in which the modulating means applies pulse waveforms to thecolumn wires according to an output in the step of making the correctionfor the influence of the voltage drop,

[0049] wherein the step of making the correction for the influence ofthe voltage drop further comprises a step of adjusting the amplitude sothat the output in the step of making the correction for the influenceof the voltage drop falls within an input range of the modulating means,and

[0050] wherein the step of converting the gradation characteristic is toselect a portion of a characteristic of canceling a light emissioncharacteristic of the electron-emitting devices in an absent state ofthe voltage drop caused by the resistance of the row wires and scanningmeans, according to an output in the step of adjusting the amplitude sothat the output falls in the input range of the modulating means.

BRIEF DESCRIPTION OF THE DRAWINGS

[0051]FIG. 1 is a diagram showing a schematic view of an image displayapparatus according to an embodiment of the present invention;

[0052]FIG. 2 is a diagram showing electrical connections of a displaypanel;

[0053]FIG. 3 is a graph showing the characteristics of surfaceconduction electron-emitting devices;

[0054]FIG. 4 is a diagram showing a driving method of the display panel;

[0055]FIGS. 5A, 5B, and 5C are diagrams for explaining a degeneratemodel;

[0056]FIG. 6 is a graph showing voltage drop amounts calculateddiscretely;

[0057]FIG. 7 is a graph showing change amounts of emission currentscalculated discretely;

[0058]FIGS. 8A, 8B, and 8C are diagrams for explaining anothercalculation method of correction data;

[0059]FIGS. 9A, 9B, and 9C are diagrams showing a calculation example ofcorrection data in the case where the size of image data is 128;

[0060]FIGS. 10A, 10B, and 10C are diagrams showing a calculation exampleof correction data in the case where the size of image data is 192;

[0061]FIGS. 11A and 11B are diagrams for explaining an interpolationmethod of correction data;

[0062]FIG. 12 is a block diagram showing a schematic configuration of animage display apparatus incorporating a gradation converter in a firstembodiment;

[0063]FIG. 13 is a block diagram showing a configuration of a scanningcircuit in the image display apparatus;

[0064]FIG. 14 is a block diagram showing a configuration of an inverse γprocessor in the image display apparatus;

[0065]FIG. 15 is a block diagram showing a configuration of a datasequence converter in the image display apparatus;

[0066]FIG. 16 is a diagram showing an example of continuous frames;

[0067]FIG. 17 is a graph showing sizes of image data in continuousframes;

[0068]FIGS. 18A and 18B are graphs showing gains in continuous frames;

[0069]FIG. 19 is a diagram showing gradation characteristics in the casewhere no correction is made for the voltage drop and there is nogradation converter provided;

[0070]FIG. 20 is a diagram showing charge-luminance characteristics;

[0071]FIG. 21 is a diagram showing the property of canceling saturationof phosphors in the case where no overflow process is carried out;

[0072]FIG. 22 is a diagram showing the relation between charge-luminancecharacteristics and gains;

[0073]FIG. 23 is a diagram showing the property of canceling saturationof phosphors in the case where the gain is 1;

[0074]FIG. 24 is a diagram showing the property of canceling saturationof phosphors in the case where the gain is ½;

[0075]FIG. 25 is a diagram showing the property of canceling saturationof phosphors in the case where the gain is ¼;

[0076]FIG. 26 is a block diagram showing a configuration example 1 ofthe gradation converter;

[0077]FIG. 27 is a block diagram showing a configuration example 2 ofthe gradation converter;

[0078]FIGS. 28A, 28B, and 28C are diagrams to illustrate the structureand operation of a modulator in the image display apparatus;

[0079]FIG. 29 is a timing chart of the modulator in the image displayapparatus;

[0080]FIG. 30 is a block diagram showing a configuration of a correctiondata calculator in the image display apparatus;

[0081]FIGS. 31A and 31B are block diagrams showing a configuration of adiscrete correction data calculating unit in the image displayapparatus;

[0082]FIG. 32 is a block diagram showing a configuration of a correctiondata interpolating unit;

[0083]FIG. 33 is a block diagram showing a configuration of a linearapproximation unit;

[0084]FIG. 34 is comprised of FIGS. 34A, 34B and 34C showing a timingchart of the image display apparatus;

[0085]FIG. 35 is a block diagram showing a schematic configuration ofthe image display apparatus of a second embodiment;

[0086]FIG. 36 is a block diagram showing the configuration of the imagedisplay apparatus of the second embodiment with smaller-scale hardware;

[0087]FIG. 37 is a block diagram showing a configuration example of thegradation converter with smaller-scale hardware in the secondembodiment; and

[0088]FIG. 38 is a block diagram showing the configuration of theconventional image display apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0089] The preferred embodiments of the present invention will beillustratively described below in detail with reference to the drawings.It is, however, noted that the scope of the invention is by no meansintended to be limited to the dimensions, materials, shapes, relativelocations, etc. of the components described in the embodiments unlessotherwise stated in particular by specific description.

[0090] (First Embodiment)

[0091] (General outline)

[0092] The display apparatus in which cold-cathode devices are arrangedin a passive matrix, experiences the phenomenon in which the voltagedrop occurs because of currents flowing into the scanning wiring and thewiring resistance of the scanning wiring, so as to degrade the displayimage. Therefore, the image display apparatus according to theembodiment of the present invention is provided with a processingcircuit configured to make appropriate correction for the influence ofthe voltage drop in the scanning wiring on the display image andsubstantiate it in a relatively small circuit scale.

[0093] The correction circuit is a circuit that estimates thedegradation of the display image caused by the voltage drop according tothe input image data, determines the correction data to correct it, andmakes correction for the input image data.

[0094] Inventors conducted elaborate research on the image displayapparatus of the type as described below, as the image display apparatusincorporating such a correction circuit.

[0095] The following will first describe the schematic of the displaypanel of the image display apparatus according to the embodiment of thepresent invention, the electrical connection of the display panel, thecharacteristics of surface conduction electron-emitting devices, adriving method of the display panel, the mechanism of the voltage dropdue to the electrical resistance of the scanning wiring, and acorrection method and apparatus for the influence of the voltage drop.

[0096] (Schematic of Image Display Apparatus)

[0097]FIG. 1 is a perspective view of the display panel used in theimage display apparatus according to the present embodiment, in whichpart of the panel is cut away in order to show the interior structure.In the drawing, numeral 1005 designates a rear plate, 1006 a side wall,and 1007 a face plate; these members 1005 to 1007 form an airtightcontainer for maintaining the interior of the display panel in vacuum.

[0098] A substrate 1001 is fixed to the rear plate 1005, and N×Mcold-cathode devices 1002 are formed on the substrate. The row wires(scanning wires) 1003, column wires (modulation wires) 1004, andcold-cathode devices are connected as shown in FIG. 2.

[0099] This wire connection structure is called a passive matrix.

[0100] A fluorescent film 1008 is formed on the bottom surface of theface plate 1007. Since the image display apparatus of the presentembodiment is a color display apparatus, phosphors of the three primarycolors, red, green, and blue, which are used in the field of CRT, areseparately deposited in the section of the fluorescent film 1008. Thephosphors are arranged to form pixels located at positions irradiatedwith electrons (emission currents) emitted from the cold-cathode devicesand formed in a matrix pattern corresponding to the respective pixels(sub-pixels) of the rear plate 1005.

[0101] A metal back 1009 is formed on the bottom surface of thefluorescent film 1008.

[0102] Hv represents a high-voltage terminal which is electricallyconnected to the metal back 1009. When a high voltage is applied to theHv terminal, a high voltage is placed between the rear plate 1005 andthe face plate 1007.

[0103] The present embodiment employs surface conductionelectron-emitting devices as the cold-cathode devices in the displaypanel as described above. It is also possible to use field emission typedevices as the cold-cathode devices. The present invention can also beapplied to the image display apparatus in which self-emitting deviceslike EL devices except for the cold-cathode devices are connected inmatrix wiring and driven.

[0104] (Characteristics of Surface Conduction Electron-emitting Devices)

[0105] The surface conduction electron-emitting devices have the(emission current Ie) versus (device voltage Vf) characteristics and the(device current If) versus (device voltage Vf) characteristics as shownin FIG. 3. Since the emission current Ie is extremely smaller than thedevice current If and it is difficult to illustrate them in an identicalscale, the two graphs are illustrated in their respective scalesdifferent from each other.

[0106] The surface conduction electron-emitting devices have the threeproperties, described below, as to the emission current Ie.

[0107] First, the emission current Ie quickly increases as the voltageapplied to the device increases over a certain voltage (which will becalled a threshold voltage Vth). On the other hand, the emission currentIe is almost zero at voltages below the threshold voltage Vth.

[0108] Namely, the devices are nonlinear devices having the distinctthreshold voltage Vth against the emission current Ie.

[0109] Second, the emission current Te varies depending upon the voltageVf applied to the devices, and it is thus feasible to control themagnitude of the emission current Ie, by varying the voltage Vf.

[0110] Third, the cold-cathode devices have the quick response property,and it is thus feasible to control the emission time of the emissioncurrent Ie by the duration of application of the voltage Vf.

[0111] By making use of the properties as described above, it becomesfeasible to suitably apply the surface conduction electron-emittingdevices to the display apparatus. For example, in the image displayapparatus using the display panel shown in FIG. 1, display can beimplemented on the basis of sequential scanning of the display screen bymaking use of the first property. Namely, voltages above the thresholdvoltage Vth according to desired emission luminances are properlyapplied to devices in driving, while a voltage below the thresholdvoltage Vth is applied to devices in a non-selected state. Bysequentially switching the driven devices, display can be implemented onthe basis of sequential scanning of the display screen.

[0112] By making use of the second property, emission luminances of thephosphors can be controlled by the voltages Vf applied to the devices,thereby enabling the image display.

[0113] By making use of the third property, light emission durations ofthe phosphors can be controlled by durations of application of thevoltages Vf to the devices, thereby enabling the display of image.

[0114] In the image display apparatus according to the embodiment of thepresent invention, modulation was implemented using the above thirdproperty.

[0115] (Driving Method of Display Panel)

[0116]FIG. 4 is an example of voltages applied to voltage supplyterminals of scanning wires and modulation wires on the occasion ofdriving the display panel of the image display apparatus according tothe embodiment of the present invention.

[0117] Now let us suppose that a horizontal scanning period I stands fora duration of light emission from pixels in the ith row.

[0118] For light emission from the pixels in the ith row, the scanningwire of the ith row is brought into a selected state and a selectionpotential Vs is impressed to its voltage supply terminal Dxi. Thevoltage supply terminals Dxk (k=1, 2, . . . N, provided that k≠i) of theother scanning wires are in a non-selected state and a non-selectionpotential Vns is impressed thereto.

[0119] In the present example, the selection potential Vs is set at −0.5V_(SEL), which is half of the voltage V_(SEL) illustrated in FIG. 3, andthe non-selection potential Vns is the GND potential.

[0120] Pulse width modulation signals with the voltage amplitude Vpwmare supplied to the voltage supply terminals of the modulation wires.Conventionally, in the case of no correction being made, a pulse widthof a pulse width modulation signal supplied to the jth modulation wirewas determined according to the size of image data for the pixel of theith row and the jth column in the display image, and pulse widthmodulation signals according to sizes of image data of the respectivepixels were supplied to all the modulation wires.

[0121] In the embodiment of the present invention, as described later,in order to make correction for the lowering of luminance due to theinfluence of the voltage drop, a pulse width of a pulse width modulationsignal supplied to the jth modulation wire is determined according tothe size of image data for the pixel of the ith row and the jth columnin the display image and a correction amount therefor, and pulse widthmodulation signals thus determined are supplied to all the modulationwires.

[0122] In the present embodiment, the voltage Vpwm is set at +0.5V_(SEL).

[0123] The surface conduction electron-emitting devices emit electronsduring application of the voltage V_(SEL) at the both ends of thedevices, but emit no electron during application of any voltage smallerthan Vth, as shown in FIG. 3.

[0124] There is also such a feature that the voltage Vth is larger than0.5 V_(SEL), as shown in FIG. 3.

[0125] For this reason, the surface conduction electron-emitting devicesconnected to the scanning wires under application of the non-selectionpotential Vns emit no electron.

[0126] Similarly, during a period in which the output of the pulse widthmodulator is the ground potential (which will be hereinafter referred toas a period of output “L”), the voltage applied to the two ends of eachsurface conduction electron-emitting device on the selected scanningwire is Vs, so that no electron is emitted therefrom.

[0127] Each surface conduction electron-emitting device on a scanningwire to which the selection potential Vs is applied, emits electronsaccording to a period in which the output of the pulse width modulatoris Vpwm (which will be hereinafter referred to as a period of output“H”) When electrons are emitted, the corresponding phosphor describedpreviously emits light according to the amount of the beam of emittedelectrons, so that light emission can be implemented at a luminanceaccording to the duration of electron emission.

[0128] The image display apparatus according to the embodiment of thepresent invention displays an image by the line sequential scanning andpulse width modulation as described above.

[0129] (Voltage Drop in Scanning Wiring)

[0130] As described above, the essential problem of the image displayapparatus is that the voltage drop in the scanning wiring of the displaypanel increases the potential on the scanning wiring, so as to decreasethe voltage applied to each surface conduction electron-emitting deviceand thus lower the emission current from the surface conductionelectron-emitting device. The mechanism of this voltage drop will bedescribed below.

[0131] Although it may differ depending upon design specifications andproduction processes of the surface conduction electron-emittingdevices, the device current from one of the surface conductionelectron-emitting devices is approximately several hundred μA duringapplication of the voltage V_(SEL).

[0132] For this reason, in the case where only one pixel on a scanningline selected in a certain horizontal scanning period is made to emitlight while the other pixels are kept off, the device current flowingfrom the modulation wire into the scanning wire of the selection row isjust the current of one pixel (i.e., aforementioned several hundred μA),and the voltage drop is almost zero, so as to cause no decrease ofemission luminance.

[0133] However, in the case where all the pixels in a selected row aremade to emit light in a certain horizontal scanning period, currents ofall the pixels flow from all the modulation wires into the scanning wirein the selected state, so that the total current becomes several hundredmA to several A, which used to cause the voltage drop on the scanningwire because of the wiring resistance of the scanning wire.

[0134] When the voltage drop occurs on the scanning wire, voltagesapplied to the two ends of the surface conduction electron-emittingdevices are lowered. This resulted in decreasing the emission currentsfrom the surface conduction electron-emitting devices, so as to decreasethe emission luminances.

[0135] To make matters more complex, the voltage drop has the naturethat the magnitude thereof also varies even during one horizontalscanning period because of the modulation based on the pulse widthmodulation.

[0136] Let us consider a case where the pulse width modulation signalssupplied to the respective columns are those with pulse widths dependingupon sizes of data and with a synchronized rise, for the input data asshown in FIG. 4. In this case, it is general, though it depends upon theinput image data, that within one horizontal scanning period, the numberof lighted pixels increases toward immediately after the rise of pulsesand the number of lighted pixels decreases with time within onehorizontal scanning period, because the pixels become unlighted in orderfrom the lowest luminance part after the rise.

[0137] Accordingly, the magnitude of the voltage drop on the scanningwire is the greatest in the initial part of one horizontal scanningperiod and tends to decrease gradually thereafter.

[0138] Since outputs of pulse width modulation signals vary at timeintervals corresponding to respective gradation levels of modulation,the voltage drop also changes with time at time intervals equivalent tothe respective gradation levels of the pulse width modulation signals.

[0139] The above described the voltage drop on the scanning wiring.

[0140] (Calculation Method of Voltage Drop)

[0141] The following will detail a method of making correction for theinfluence of the voltage drop.

[0142] The first step necessary for determining correction amounts forreducing the influence of the voltage drop is to develop hardwarecapable of estimating the magnitude of the voltage drop and the temporalchange thereof in real time.

[0143] However, the display panel of the image display apparatus as inthe present invention generally has several thousand modulation wiresand it is very difficult to calculate voltage drops at intersectionsbetween all the modulation wires and a scanning wire. Therefore, it isnot so practical to fabricate hardware for calculating them in realtime.

[0144] Thus the voltage drop amounts are determined by grouping thedevices into blocks as to positions on the same row and also groupingimage data into blocks as to amplitudes thereof.

[0145] This grouping into blocks is based on the following features ofthe voltage drops.

[0146] i) At a certain point of time in one horizontal scanning period,the voltage drops appearing on the scanning wire are spatiallycontinuous amounts on the scanning wire and are represented by a verysmooth curve.

[0147] ii) The magnitudes of voltage drops, which differ depending uponthe display image, vary at time intervals equivalent to the respectivegradation levels of the pulse width modulation; schematically, they arelarge at the rise of pulses, and they gradually decrease with time orare maintained.

[0148] Namely, in the driving method as shown in FIG. 4, the magnitudesof voltage drops never increase within one horizontal scanning period.

[0149] Specifically, the time change of voltage drops was schematicallyestimated by calculating the voltage drops at a plurality of times onthe basis of a degenerate model described below.

[0150] (Calculation of Voltage Drops by Degenerate Model)

[0151]FIG. 5A is a diagram for explaining blocks and nodes employed indegeneration.

[0152] For simplicity of illustration, FIG. 5A shows only a selectedscanning wire, modulation wires, and surface conductionelectron-emitting devices coupled at intersections between the wires.

[0153] It is assumed herein that the present time is a certain time inone horizontal scanning period and lighting conditions of the respectivepixels on the selected scanning wire (i.e., whether the output of themodulator is “H” or “L” for each device) are known.

[0154] In this lighting state, device currents flowing from therespective modulation wires into the selected scanning wire are definedas Ifi (i=1, 2, . . . N, where i represents a column number).

[0155] As shown in the same figure, a block is defined as one groupincluding n modulation wires, a portion of the selected scanning wireintersecting therewith, and surface conduction electron-emitting devicesplaced at intersections between them. In the present example, the partof interest was divided into four blocks by grouping into blocks.

[0156] Positions denoted by nodes are defined at the boundary positionsof the respective blocks. The nodes are horizontal positions (referencepoints) for discretely calculating the voltage drop amounts appearing onthe scanning wire in the degenerate model.

[0157] In the present example five nodes, node 0 to node 4, are set atthe boundary positions of the blocks.

[0158]FIG. 5B is a diagram for explaining the degenerate model.

[0159] In the degenerate model n modulation wires included in one blockin FIG. 5A are degenerated into one, and one degenerate modulation wireis connected so as to be located in the center of each block of thescanning wire.

[0160] It is assumed that a current source is connected to a modulationwire of each degenerate block and summations IF0-IF3 of currents in therespective blocks flow from the respective current sources into themodulation wires.

[0161] Namely, IFj (j=0, 1, . . . 3) represents the electric currentexpressed by the equation below. $\begin{matrix}{{IFj} = {\sum\limits_{i = {{j \times n} + 1}}^{{({j + 1})} \times n}{Ifi}}} & \left( {{Eq}\quad 1} \right)\end{matrix}$

[0162] The potential at the both ends of the scanning wire is Vs in theexample of FIG. 5A, whereas it is the GND potential in FIG. 5B. Thereason for it is that in the degenerate model the electric currentsflowing from the modulation wires into the selected scanning wire aremodeled by the above current sources and thus the voltage drop amountsof the respective portions on the scanning wire can be calculated bydetermining voltages (potential differences) of the respective portionswith respect to the power supply part at the reference (GND) potential.

[0163] Namely, the ground potential is defined as a reference potentialin calculation of voltage drops.

[0164] The reason why the surface conduction electron-emitting devicesare omitted is that, as far as the selected scanning wire is concerned,the presence or absence of the surface conduction electron-emittingdevices makes no change in the appearing voltage drops themselves ifequivalent electric currents flow from the column wires. Therefore, thesurface conduction electron-emitting devices are omitted herein whilethe values of currents flowing from the current sources of therespective blocks are set at the current values (Eq 1) equal to thesummations of the device currents in the respective blocks.

[0165] A wiring resistance of the scanning wire of each block is definedas n times a wiring resistance r of the scanning wire in one interval(one interval herein refers to a zone between an intersection of thescanning wire with a certain column wire and an intersection of thescanning wire with a column wire adjacent thereto and it is also assumedin the present example that the wiring resistance of the scanning wireis uniform throughout each interval).

[0166] In the degenerate model described above, the voltage drop amountsDV0 to DV4 appearing at the respective nodes on the scanning wire can bereadily calculated by equations below in the form of sum of products.

DV 0=a 00×IF 0+a 01×IF 1+a 02×IF 2+a 03×IF 3

DV 1=a 10×IF 0+a 11×IF 1+a 12×IF 2+a 13×IF 3

DV 2=a 20×IF 0+a 21×IF 1+a 22×IF 2+a 23×IF 3

DV 3=a 30×IF 0+a 31×IF 1+a 32×IF 2+a 33×IF 3

DV 4=a 40×IF 0+a 41×IF 1+a 42×IF 2+a 43×IF 3

[0167] Namely, the following equation holds. $\begin{matrix}{{DVi} = {\sum\limits_{j = 0}^{3}{{aij} \times {{IFj}\left( {{i = 0},1,2,3,{{or}\quad 4}} \right)}}}} & \left( {{Eq}\quad 2} \right)\end{matrix}$

[0168] In the above equation, aij represents a voltage appearing at theith node when a unit current is injected into only the jth block in thedegenerate model (which will be defined hereinafter as aij).

[0169] The above aij is derived by the Kirchhoff's law and the result ofthe calculation thereof once performed can be stored in the form of atable.

[0170] Furthermore, the approximation as expressed by Eq 4 below iseffected for the summation currents IF0-IF3 of the respective blocksdefined in Eq 1. $\begin{matrix}{{IFj} = {{\sum\limits_{i = {{j \times n}\quad + 1}}^{{({j + 1})} \times n}{Ifi}} = {{IFS} \times {\sum\limits_{i = {{j \times n} + 1}}^{{({j + 1})} \times n}{{Count}\quad i}}}}} & \left( {{Eq}\quad 4} \right)\end{matrix}$

[0171] In the above equation, Count i represents a variable that takes 1when the ith pixel on the selected scanning line is in a lighted stateand that takes zero when the ith pixel is in an unlighted state.

[0172] IFS indicates a quantity obtained by multiplying a device currentIF flowing during application of the voltage V_(SEL) at the two ends ofone of the surface conduction electron-emitting devices, by a factor ataking a value between 0 and 1.

[0173] Namely, it is defined as follows.

IFS=α×IF  (Eq 5)

[0174] Eq 4 is based on the assumption that a device currentproportional to the number of lighting devices in each block flows froma column wire of the block into the selected scanning wire. Thefollowing is the reason why the product of the device current IF of onedevice and the coefficient α is defined as the device current IFS of onedevice. In order to calculate a voltage drop amount, it is originallynecessary to repeatedly calculate a voltage increase of the scanningwire due to a voltage drop and a decrease amount of the device currentcaused thereby, but it is not practical to perform this convergencecalculation by hardware. Therefore, the present invention employsapproximate αIF as a convergence value of IF. Specifically, the factoris determined as follows: preliminarily estimated are a decrease rate ofIF at the maximum voltage drop amount (with all devices turn-on) (=α1)and a decrease rate of IF at the minimum voltage drop amount (minimum=0)(=α2); and it is then determined as an average of α1 and α2 or as0.8×α1.

[0175]FIG. 5C is an example of the result of calculation of voltage dropamounts DV0-DV4 at the respective nodes by the degenerate model, in acertain lighting state.

[0176] Since the voltage drop is given by a very smooth curve, thevoltage drop between nodes is assumed to take approximate values asindicated by a dotted line in the drawing.

[0177] As described above, the use of the present degenerate modelpermits the voltage drop to be calculated at the positions of the nodesand at any desired point of time for the input image data.

[0178] As described above, the voltage drop amounts in a certainlighting state are simply calculated using the degenerate model.

[0179] The voltage drop appearing on the selected scanning wire varieswith time in one horizontal scanning period, and this temporal changewas estimated in such a manner that lighting states were determined atseveral points of time in one horizontal scanning period, as describedpreviously, and voltage drops in the respective lighting states weredetermined using the degenerate model.

[0180] The number of lighting devices in each block at a certain pointof time in one horizontal scanning period can be readily determined bymaking reference to the image data of each block.

[0181] Let us suppose here that the bit count of input data into thepulse width modulator is 8 bits as an example and the pulse widthmodulator provides outputs of pulse widths according to sizes of theinput data.

[0182] Namely, it is assumed that with the input data of 0, the outputis “L”; with the input data of 255, the output is “H” throughout onehorizontal scanning period; with the input data of 128, the output is“H” during the first half of one horizontal scanning period and “L”during the second half.

[0183] In this case, the number of lighting devices at the time of thestart of pulse width modulation signals (at the time of the rise in theexample of the modulation signals in the present example) can be readilydetected by counting the input data into the pulse width modulatorgreater than 0.

[0184] Likewise, the number of lighting devices at the time of themiddle of one horizontal scanning period can be readily detected bycounting the input data into the pulse width modulator greater than 128.

[0185] In this way the number of lighting devices at an arbitrary timecan be readily calculated by comparing the image data with a certainthreshold in a comparator and counting true outputs of the comparator.

[0186] For simplification of the description hereinafter, let us definea time slot as a time quantity.

[0187] Namely, the time slot indicates a time from the rise of the pulsewidth modulation signals in one horizontal scanning period, and the timeslot=0 is defined as one indicating a time immediately after the time ofthe start of a pulse width modulation signal.

[0188] The time slot=64 is defined as a time elapsed for a duration ofsixty four gradation levels from the time of the start of a pulse widthmodulation signal.

[0189] The present example provided the example in which the pulsewidths were modulated with respect to the reference at the time of therise. It is needless to mention that the present invention can also besimilarly applied to the case where the pulse widths are modulated withrespect to the reference at a time of a fall of pulses, though theadvancing direction of the time axis becomes reverse to the advancingdirection of the time slot.

[0190] (Calculation of Correction Data from Voltage Drop Amounts)

[0191] As described above, we succeeded in approximately and discretelycalculating the temporal change of the voltage drop during onehorizontal scanning period by the iterative calculation using thedegenerate model.

[0192]FIG. 6 is an example in which the temporal change of the voltagedrop on the scanning wire was calculated on the basis of the iterativecalculation of voltage drop for certain image data. (It is noted thatthe voltage drop and the temporal change thereof presented herein arejust an example for certain image data and it is a matter of course thatthe voltage drop for another image data demonstrates another change.)

[0193]FIG. 6 shows the result of calculation in which the voltage dropamounts at the respective times were discretely calculated by applyingthe degenerate model to each of four time points of time slots=0, 64,128, and 192.

[0194] In FIG. 6 the voltage drop amounts at the respective nodes areconnected by dotted lines, and it is noted that the dotted lines arepresented for a better look of illustration. In fact, the voltage dropamounts calculated by the degenerate model were discretely calculated atthe positions of each node as indicated by □, ∘, , and Δ.

[0195] Inventors conducted research on a method of calculating thecorrection data for correction for image data from the voltage dropamounts, as the next stage after the implementation of calculation ofthe magnitude and temporal change of voltage drop.

[0196]FIG. 7 is a graph of estimated amounts of emission current emittedfrom the surface conduction electron-emitting devices in the lightedstate under the condition that the voltage drop shown in FIG. 6 occurredon the selected scanning wire.

[0197] The vertical axis indicates an amount of emission current at eachtime and at each position in percentage with respect to 100% as anamount of emission current emitted in the case of no voltage drop, andthe horizontal axis the horizontal position.

[0198] As shown in FIG. 7, let us define emission currents at thehorizontal position of node 2 (reference point) as follows:

[0199] Ie0: emission current at the time slot=0;

[0200] Ie1: emission current at the time slot=64;

[0201] Ie2: emission current at the time slot=128;

[0202] Ie3: emission current at the time slot=192.

[0203]FIG. 7 shows the result of calculation from the graphs of thevoltage drop amounts of FIG. 6 and the “drive voltage-emission current”of FIG. 3. Specifically, it is a mechanical plot of values of emissioncurrent during application of voltages obtained by subtracting thevoltage drop amounts from the voltage V_(SEL).

[0204] Accordingly, FIG. 7 consistently shows the currents emitted fromthe surface conduction electron-emitting devices in the lighted state,and the surface conduction electron-emitting devices in the unlightedstate emit no current.

[0205] The following will describe a method of calculating thecorrection data for correction for image data from the voltage dropamounts.

[0206] (Correction Data Calculating Method)

[0207]FIGS. 8A, 8B, and 8C are diagrams for explaining a method ofcalculating correction data of a voltage drop amount from the temporalchange of emission current shown in FIG. 7. These figures show anexample of the calculation of the correction data for the image data inthe size of 64.

[0208] The quantity of emitted light with a luminance is nothing but theamount of emitted charge which is obtained by integrating the emissioncurrent during an emission current pulse over time. For consideringvariation of luminance due to the voltage drop, therefore, thedescription hereinafter will be given on the basis of the amount ofemitted charge.

[0209] Let IE be an emission current in the case of no influence of thevoltage drop, and Δt be a time equivalent to one gradation level of thepulse width modulation. Then an emitted charge amount Q0 of charge to beemitted by an emission current pulse for the image data of 64 can beexpressed as follows by multiplying the amplitude IE of the emissioncurrent pulse by the pulse width (64×Δt).

Q 0=IE×64×Δt  (Eq 6)

[0210] In practice, however, there occurs the phenomenon of decrease ofemission current due to the voltage drop on the scanning wire.

[0211] The emitted charge amount with the emission current pulse takingaccount of the influence of the voltage drop can be calculatedapproximately as follows. Namely, let Ie0 and Ie1 be emission currentsin the respective time slots=0 and 64 at the node 2, and let us assumethat the emission current between 0 and 64 is approximated so as tolinearly change between Ie0 and Ie1. Then the emitted charge amount Q1during this period is given by the area of the trapezoid shown in FIG.8B.

[0212] Namely, it can be calculated as follows.

Q 1=(Ie 0+Ie 1)×64×Δt×0.5  (Eq 7)

[0213] Let us then suppose that, as shown in FIG. 8C, the influence ofthe voltage drop was successfully eliminated when the pulse width wasextended by DC1 in order to make correction for the decrease of theemission current due to the voltage drop.

[0214] It is considered that the emission current amount in each timeslot varies when the pulse width is extended for the correction for thevoltage drop. For simplification, however, it is assumed herein that theemission current is Ie0 in the time slot=0 and the emission current Ie1in the time slot=(64+DC1), as shown in FIG. 8C.

[0215] An approximation is also made so that the emission currentbetween the time slot 0 and the time slot (64+DC1) takes values on astraight line connecting the emission currents at the two points.

[0216] Then, an emitted charge amount Q2 with an emission current pulseafter the correction can be calculated as follows.

Q 2=(Ie 0+Ie 1)×(64+DC 1)×Δt×0.5  (Eq 8)

[0217] By equating this to aforementioned Q0, we obtain the following.

IE×64×Δt=(Ie 0+Ie 1)×(64+DC 1)×Δt×0.5

[0218] By solving this with respect to DC1, we obtain the following.

DC 1=((2×IE−Ie 0−Ie 1)/(Ie 0+Ie 1))×64  (Eq 9)

[0219] In this way, the correction data was calculated for the case ofthe image data being 64.

[0220] Namely, for the image data in the size of 64 at the position ofthe node 2, the correction amount CData to be added can be given byCData=DC1, as represented by Eq 9.

[0221] Likewise, a correction amount can be determined for each of twoperiods, as shown in FIGS. 9A to 9C, for the image data in the size of128, and a correction amount can be determined for each of threeperiods, as shown in FIGS. 10A to 10C, for the image data in the size of192.

[0222] Since the influence of the voltage drop on the emission currentis, of course, null with the pulse width of 0, the correction data wasset as 0 and the correction data CData to be added to the image data wasalso set as 0.

[0223] The reason why the correction data was calculated for discreteimage data like 0, 64, 128, and 192 as described above, is that it canreduce the calculation amount.

[0224]FIG. 11A shows an example of discrete correction data for certaininput image data, which was obtained by the present method. In the samefigure the horizontal axis represents the horizontal display positionand the positions of the respective nodes are illustrated. The verticalaxis represents the size of correction data.

[0225] The discrete correction data was calculated at the positions ofthe nodes and for the sizes of the image data Data (image data referencevalues=0, 64, 128, and 192) as indicated by □, ∘, , and Δ in thedrawing.

[0226] (Interpolation Method of Discrete Correction Data)

[0227] The correction data discretely calculated is discrete data at thepositions of the respective nodes, and does not give correction data atan arbitrary horizontal position (column wiring number). At the sametime as it, the correction data is data for image data in severalpredetermined sizes of reference values of image data at each nodeposition, but does not give correction data according to the actualsizes of image data.

[0228] Then Inventors calculated the correction data fitting sizes ofinput image data in the respective column wires by interpolation of thediscretely calculated correction data.

[0229]FIG. 11B is a diagram showing a method of calculating thecorrection data for the image data Data at x located between the node nand the node (n+1).

[0230] As a premise, the correction data has already discretely beencalculated at the positions Xn and Xn+1 of the node n and the node n+1.

[0231] It is also assumed that the input image data Data takes valuesbetween the image data reference values Dk and Dk+1.

[0232] Letting CData[k][n] be the discrete correction data for thereference value of the kth image data at the node n, the correction dataCA for the pulse width Dk at the position x can be calculated as followsby linear approximation, using the values of CData[k][n] andCData[k][n+1].

[0233] Namely, the correction data CA is given as follows.$\begin{matrix}{{CA} = \frac{{\left( {{Xn} + 1 - x} \right) \times {{{CData}\lbrack k\rbrack}\lbrack n\rbrack}} + {\left( {x - {Xn}} \right) \times {{{CData}\lbrack k\rbrack}\left\lbrack {n + 1} \right\rbrack}}}{{Xn} + 1 - {Xn}}} & \left( {{Eq}\quad 17} \right)\end{matrix}$

[0234] In this equation, Xn and Xn+1 indicate the horizontal displaypositions of the respective nodes n and (n+1), which are constantssettled in the determination of the aforementioned blocks.

[0235] The correction data CB for the image data Dk+1 at the position xcan be calculated as follows.

[0236] Namely, the correction data CB is given by the followingequation. $\begin{matrix}{{CB} = \frac{{\left( {{Xn} + 1 - x} \right) \times {{{CData}\left\lbrack {k + 1} \right\rbrack}\lbrack n\rbrack}} + {\left( {x - {Xn}} \right) \times {{{CData}\left\lbrack {k + 1} \right\rbrack}\left\lbrack {n + 1} \right\rbrack}}}{{Xn} + 1 - {Xn}}} & \left( {{Eq}\quad 18} \right)\end{matrix}$

[0237] By linear approximation with the correction data CA and CB, thecorrection data CD for the image data Data at the position x can becalculated as follows.

[0238] Namely, the correction data CD is given by the followingequation. $\begin{matrix}{{CD} = \frac{{{CA} \times \left( {{Dk} + 1 - {Data}} \right)} + {{CB} \times \left( {{Data} - {Dk}} \right)}}{{Dk} + 1 - {Dk}}} & \left( {{Eq}\quad 19} \right)\end{matrix}$

[0239] As described above, the correction data matching the actualpositions and sizes of image data can be readily calculated from thediscrete correction data by the method as described by Eq 17 to Eq 19.

[0240] The dashed lines connecting the nodes in FIG. 11A are the resultsof interpolation from the discrete correction data by the abovecalculation. As seen from the figure, the voltage drop correction methodof the present invention results in yielding the same correction datafor all the positions x (also including the correction data of 0, ofcourse), because there occurs no voltage drop in the case of the imagedata of 0; and it results in obtaining the correction data in a gentlycurved distribution for the positions x, i.e., the horizontal directionof the screen in the case of the same image data being not equal to 0.It is, however, noted that, in the case of the scanning lines beingdirected in the perpendicular direction in the screen, the correctiondata is one in a gently curved distribution in the perpendiculardirection in the screen.

[0241] By correcting the image data by adding the correction data thuscalculated, to the image data and effecting the pulse width modulationaccording to the image data after the correction (referred to ascorrected image data), it becomes feasible to reduce the influence ofthe voltage drop on the display image, which was the problem heretofore,and thereby improve the quality of image.

[0242] There is also the excellent advantage that the hardware for thecorrection, which was the problem heretofore, can be constructed in verysmall scale, because the calculation amount can be decreased byintroduction of the approximations including the degeneracy as describedabove.

[0243] (Description of Entire System and Functions of RespectivePortions)

[0244] The following will describe the hardware of the image displayapparatus incorporating the correction data calculator.

[0245]FIG. 12 is a block diagram schematically showing a circuitconfiguration of the apparatus. In the drawing, numeral 1 designates thedisplay panel of FIG. 1; Dx1-DxM and Dx1′-DxM′ voltage supply terminalsof the scanning wires of the display panel; Dy1-DyN voltage supplyterminals of the modulation wires of the display panel; Hv ahigh-voltage supply terminal for placing an acceleration voltage betweenthe face plate and the rear plate; Va a high voltage power source; 2 and2′ scanning circuits; 3 a synchronizing signal separator; 4 a timinggenerator; 7 a converter for converting a YPbPr signal separated by thesynchronizing signal separator 3, into RGB signals; 23 a selector forimplementing changeover between a television video signal and a computervideo signal; 17 an inverse γ processing unit; 5 a shift register forone line of image data; 6 a latch for one line of image data; 8 a pulsewidth modulator for outputting modulation signals into the modulationwires of the display panel; 12 an adder; 14 a correction datacalculator; 20 a maximum value detector; 21 a gain calculator; 200 agradation converter.

[0246] Since the gradation converter 200 will be described later, thedescription below will be given excluding the gradation converter 200.

[0247] In the same drawing, R, G, and B represent parallel RGB inputvideo data; Ra, Ga, and Ba parallel RGB video data subjected to inverseγ conversion processing described hereinafter; Data image data obtainedby parallel-serial conversion at the data sequence converter; CD thecorrection data calculated by the correction data calculator; Dout thecorrected image data (adjusted image data) obtained by adding thecorrection data to the image data at the adder.

[0248] (Synchronizing Separator and Selector)

[0249] The image display apparatus of the present embodiment can displaytelevision signals of NTSC, PAL, SECAM, HDTV, etc., and VGA and the likeas computer output.

[0250] A video signal of the HDTV system is first fed into thesynchronizing separator 3, in which synchronizing signals Vsync, Hsyncare separated therefrom and are supplied into the timing generator. Thevideo signal synchronously separated is supplied into the RGB converter.Inside the RGB converter there are provided a low-pass filter, an A/Dconverter, etc., not shown, in addition to the converter from YPbPr intoRGB. The RGB converter converts the YPbPr signal into digital RGBsignals and supplies the RGB signals into the selector 23.

[0251] A video signal of VGA or the like outputted from a computer issubjected to A/D conversion at an unrepresented A/D converter and theresultant digital signal is supplied into the selector 23.

[0252] The selector 23 properly selects either of the television signaland the computer signal and outputs the signal selected based on whichvideo signal the user desires to display.

[0253] (Timing Generator)

[0254] The timing generator incorporates a PLL circuit and is a circuitfor generating timing signals compatible with various video formats andthus generating operation timing signals of the respective units.

[0255] The timing signals generated by the timing generator 4 includeTsft for control over the operation timing of the shift register 5, acontrol signal Dataload for latching of data from the shift registerinto the latch 6, a pulse width modulation start signal Pwmstart of themodulator 8, a clock Pwmclk for the pulse width modulation, Tscan forcontrol over the operation of the scanning circuits 2 and 2′, and so on.

[0256] (Scanning Circuits)

[0257] As shown in FIG. 13, the scanning circuits 2 and 2′ are circuitsfor supplying a selected voltage Vs or a non-selected voltage Vns intothe connection terminals Dx1-DxM in order to implement sequentialscanning of the display panel by one row per horizontal scanning period.

[0258] The scanning circuits 2 and 2′ are circuits for implementingscanning while sequentially switching the scanning wire selected in eachhorizontal period one from another in synchronism with the timing signalTscan from the timing generator 4.

[0259] Tscan is a timing signal group made from the verticalsynchronizing signal and horizontal synchronizing signal, for example.

[0260] Each of the scanning circuits 2 and 2′ is comprised of Mswitches, a shift register, and others as shown in FIG. 13. Theseswitches are preferably constructed of transistors or FETs.

[0261] In order to decrease the voltage drop on the scanning wires, thescanning circuits are preferably coupled to the two ends of the scanningwires of the display panel, as shown in FIG. 12, and are configured todrive the panel from the two ends.

[0262] On the other hand, the embodiment of the present invention isalso effective in the case where the scanning circuits are not coupledto the both ends of the scanning wires, and can be applied thereto byonly modifying the parameters in Eq 2.

[0263] (Inverse γ Processor)

[0264] CRT has the approximately 2.2th power emission characteristic(hereinafter referred to as an inverse γ characteristic) against input.

[0265] The input video signal takes account of this characteristic ofCRT, and is generally converted according to the 0.45th power γcharacteristic so as to achieve a linear emission characteristic indisplay on the CRT.

[0266] On the other hand, the display panel of the image displayapparatus according to the embodiment of the present invention has anapproximately linear emission characteristic against applied time in thecase of the modulation being made by applied durations of the drivingvoltage, and it is thus necessary to convert the input video signal onthe basis of the inverse γ characteristic (hereinafter referred to asinverse γ conversion).

[0267] The inverse γ processor shown in FIG. 12 is a block forimplementing the inverse γ conversion of the input video signal.

[0268] The inverse γ processor of the present embodiment is comprised ofmemories for implementing the above inverse γ conversion process.

[0269] On the assumption the bit count of the video signals R, G, and Bis 8 bits and that the bit count of the video signals Ra, Ga, and Ba asoutputs of the inverse γ processor is also 8 bits, the inverse γprocessor is comprised of memories of 8-bit address and 8-bit data forthe respective colors (FIG. 14).

[0270] (Data Sequence Converter)

[0271] The data sequence converter 9 is a circuit of performingparallel-serial conversion of the parallel RGB video signals Ra, Ga, andBa so as to fit the pixel array of the display panel. The data sequenceconverter 9 is comprised of FIFO (First In First Out) memories 2021R,2021G, and 2021B for the respective colors R, G, and B and a selector2022, as shown in FIG. 15.

[0272] Although not shown in the same figure, each FIFO memory isprovided with two memories of the word equal to the number of horizontalpixels, one for odd lines and one for even lines. With entry of videodata of an odd row, the data is written into the FIFO memories for oddlines, while the image data stored during one preceding horizontalscanning period is read out of the FIFO memories for even lines. Withentry of video data of an even row, the data is written into the FIFOmemories for even lines, while the image data stored during onepreceding horizontal period is read out of the FIFO memories for oddlines.

[0273] Data read out of the FIFO memory is subjected to parallel-serialconversion according to the pixel array of the display panel at theselector, and serial RGB image data SData is outputted therefrom. Thefurther details will not be given herein, but the operation is performedbased on the timing control signal from the timing generator 4.

[0274] (Delay Circuit 19)

[0275] The image data SData after the rearrangement at the data sequenceconverter is fed into the correction data calculator and into the delaycircuit 19. A correction data interpolation unit of the correction datacalculator, described later, makes reference to values of the horizontalposition information x from the timing control circuit and the imagedata SData to calculate correction data CD matching them.

[0276] The delay circuit 19 is a means provided for absorbing the timenecessary for the calculation of the correction data (the aforementionedinterpolation process of correction data) and configured to give such adelay as to correctly add corresponding correction data to image data onthe occasion of adding the correction data to the image data at theadder. This means can be constructed of a flip-flop circuit.

[0277] (Adder 12)

[0278] The adder 12 is a unit of adding the correction data CD from thecorrection data calculator to the image data Data. The addition makesthe correction for the image data Data and the corrected image data Doutis transferred to the maximum value detector and to the multiplier.

[0279] The bit count of the corrected image data being the output of theadder is preferably settled so as to cause no overflow in the additionof the correction data to the image data.

[0280] More specifically, let us suppose that the image data Data hasthe data width of 8 bits and the maximum thereof is 255 and that thecorrection data CD has the data width of 7 bits and the maximum thereofis 120.

[0281] In this case, the maximum of the addition result becomes255+120=375.

[0282] Under the above condition, the corrected image data Dout beingthe output of the adder is preferably output with the output bit widthof 9 bits in order to prevent the overflow.

[0283] (Overflow Process)

[0284] In the embodiment of the present invention, the correction isimplemented by the corrected image data resulting from the addition ofthe calculated correction data to the image data, as describedpreviously.

[0285] Now suppose that the bit count of the modulator is 8 bits and thebit count of the corrected image data Dout being the output of the adderis 10 bits.

[0286] Then overflow will occur if the corrected image data is suppliedto the input of the modulator as it is.

[0287] It is thus necessary to adjust the amplitude of the correctedimage data, prior to the supply into the modulator.

[0288] A potential configuration for preventing the overflow is topreliminarily estimate a maximum of the corrected image data upon entryof maximum input image data in an all-devices-turn-on pattern ((R, G,B)=(FFh, FFh, FFh) in the case of the bit count of the image data being8 bits) and multiply the corrected image data by a gain so as to keep itwithin the input range of the modulator.

[0289] This method will be referred to hereinafter as a fixed gainmethod.

[0290] The fixed gain method does not cause the overflow, but isconfigured to multiply the corrected image data for an image with a lowaverage luminance by the small gain, though it can be displayed with agreater gain. Therefore, the display image can be dark in luminance.

[0291] In contrast to it, another potential method for preventing theoverflow is a method of detecting a maximum of the corrected image datain every frame, calculating a gain so as to keep the maximum within theinput range of the modulator, and multiply the corrected image data bythe gain.

[0292] This method will be referred to hereinafter as an adaptive gainmethod.

[0293] The adaptive gain method necessitates the maximum value detector20 for detecting the maximum MAX of the corrected image data Dout inevery frame, the gain calculator 21 for calculating the gain G1 formultiplication with the corrected image data from the maximum, and amultiplier for multiplying the corrected image data Dout by the gain G1.

[0294] In the adaptive gain method, the gain for preventing the overflowis preferably calculated in frame units.

[0295] It is also possible to calculate the gain for every horizontalline and prevent the overflow, for example. This configuration is,however, not preferable, because the display image looks strange becauseof the difference among the gains of the respective horizontal lines.

[0296] The above schematically described the fixed gain method and theadaptive gain method.

[0297] Inventors confirmed that the amplitude of the corrected imagedata could be suitably adjusted when the gain was calculated by eitherof the methods.

[0298] The present embodiment is thus configured to implement theadjustment of amplitude by the adaptive gain method.

[0299] The following will detail a circuit configuration forimplementing the adjustment of amplitude of the corrected image data bythe adaptive gain method in the present embodiment.

[0300] (Maximum Detector 20)

[0301] The maximum detector 20 of the present invention is connected toeach of the units, as shown in FIG. 12.

[0302] The maximum detector 20 is a unit of detecting a maximum valueout of the corrected image data Dout of one frame.

[0303] This unit is a circuit that can be readily constructed of acomparator and a register, for example. This unit is a circuit ofcomparing the size of the corrected image data sequentially transferred,with a value stored in the register, and updating the value of theregister with the data value if the corrected image data is greater thanthe value of the register.

[0304] The value of the register is cleared to 0 at the head of eachframe, and then a maximum value of the corrected image data in a frameis stored in the register at the time of the end of that frame.

[0305] The maximum value of the corrected image data detected in thisway is transferred to the gain calculator 21.

[0306] (Gain Calculator)

[0307] The gain calculator 21 is a unit of calculating the gain for theadjustment of amplitude so as to keep the corrected image data Doutwithin the input range of the modulator on the basis of the adaptivegain method.

[0308] The gain can be successfully determined if it satisfies thefollowing condition:

gain G≦INMAX/MAX  (Eq 20),

[0309] where MAX is the maximum value detected by the maximum valuedetector 20 and INMAX the maximum value of the input range of themodulator (first method).

[0310] The gain calculator 21 updates the gain in a vertical retraceperiod to change the value of the gain every frame.

[0311] In the configuration of the image display apparatus according tothe embodiment of the present invention, the gain by which the correctedimage data of the current frame is multiplied, is calculated using themaximum value of the corrected image data of one preceding frame.

[0312] Strictly speaking, overflow may occur because of the differenceof corrected image data between frames accordingly.

[0313] To solve this problem, a circuit was designed with a limiter,described later, for the output of the multiplier for multiplying thecorrected image data by the gain, so as to keep the output of themultiplier within the input range of the modulator, and provided goodresult.

[0314] The above overflow process can be deemed as an overflow processmaking use of the correlation of corrected image data (image data)between adjacent frames.

[0315] It becomes feasible to prevent the overflow in a configurationwithout a time delay if a frame memory is provided between the maximumvalue detector and the multiplier.

[0316] Inventors confirmed that the gain determining method based on theadaptive gain method could be arranged to calculate the gain by themethod as described below.

[0317] Namely, the gain for the corrected image data of the currentframe can be suitably determined so as to satisfy the condition below:

gain G1≦INMAX/AMAX  (Eq 21),

[0318] where AMAX is an average obtained by framewise smoothing (oraveraging) of maximums of corrected image data detected in framespreceding to the current frame (second method).

[0319] A third method is a method of calculating the gains G1 of therespective frames according to Eq 20 and averaging them to obtain thecurrent gain.

[0320] Inventors confirmed that all these three methods were suitablyapplicable and that the second and third methods were exceedinglysuitable, rather than the first method, because they had another effectof largely reducing flicker in the display image (which will bedescribed later with reference to FIG. 16).

[0321] Inventors conducted research on the number of frames to be usedfor the averaging in the second method and the third method, andconfirmed that acceptable images were obtained with little flicker, forexample, by averaging of 16 to 64 frames.

[0322] Just as in the case of the first method, the second and thirdmethods can decrease the probability of occurrence of overflow becauseof the correlation of (corrected) image data between frames, but alsofail to prevent the overflow perfectly.

[0323] As countermeasures to it, we employed a method of preventing theoverflow roughly by the above method and preventing the overflowperfectly by provision of the limiter at the output of the multiplier,and obtained better result.

[0324]FIG. 16 is a diagram for explaining the flicker, using the firstmethod and the second method as an example.

[0325]FIG. 16 shows an example of a moving picture in which a white baris rotating counterclockwise over the gray background. In the case ofsuch an image being displayed, the size of the correction data CDlargely varies frame by frame with rotation of the bar.

[0326]FIG. 17 is a diagram for explaining the corrected image data inthe correction for such a moving picture. FIG. 17 is a graph of maximumsin the respective frames out of the corrected image data of therespective frames.

[0327] In FIG. 17 white portions correspond to the original image data,and gray portions to portions extended by the correction.

[0328] In the case of the display of the image as shown in FIG. 16, themaximums of the corrected image data in the continuous frames vary asshown in FIG. 17.

[0329] Therefore, if gains are set for the respective frames asindicated by Eq 20, the gains of the respective frames will vary heavilyas shown in FIG. 18A, so as to intensify the variation of luminance inthe display image, thereby raising a sense of flicker.

[0330] In contrast to it, when the gains are determined by Eq 21, thegains are averaged and thus the variation of gains becomes smaller, asshown in FIG. 18B, so as to reduce the variation of luminance, therebyachieving the excellent effect of reducing a sense of flicker.

[0331] In FIG. 18B the plot of white circles represents the gainsaccording to Eq 20, while the plot of black circles the gains averagedby Eq 21.

[0332] Although the third method was not discussed in detail herein,Inventors also confirmed that the variation of gains became smaller, soas to reduce flicker, as in the case of the second method.

[0333] It was preferable that the gain calculator 21 should beconfigured to average the gains for the screen of continuous scenes asdescribed above but quickly change the gains for the screen of differentscenes on the occasion of switching between scenes of images.

[0334] For implementing it, a preferred configuration was such that thegain calculator 21 was configured to have a preset threshold value as ascene switching threshold Gth and calculate the gain of the next frameby smoothing in such a way that:

[0335] if ΔG=|GN−GB|>Gth,

[0336] gain G1=(GN−GB)×A+GB; or

[0337] if ΔG=|GN−GB|≦Gth,

[0338] gain G1=(GN−GB)−B+GB

[0339] (where A and B are real numbers satisfying the relation of1≧A≧B>0),

[0340] where GB is a gain of one preceding frame calculated by Eq 20, GNa gain calculated by Eq 20 from the maximum of the corrected image dataof the preceding frame, detected by the maximum detector 20, and ΔG anabsolute value of the difference GN−GB.

[0341] Particularly preferable results were obtained when the values ofA and B were set as A=1 and B={fraction (1/16)} to {fraction (1/64)}approximately.

[0342] (Multiplier)

[0343] The multiplier in FIG. 12 multiplies the corrected image dataDout being the output of the adder, by the gain G1 calculated by thegain calculator, and transfers the result as the corrected image datawith the adjusted amplitude, Dmult, to the limiter.

[0344] (Limiter)

[0345] If the gain can be determined as described above so as to causeno overflow, there will arise no problem. It is, however, difficult todetermine the gain with no overflow at all by the several gaindetermining methods described above. Therefore, the apparatus may alsobe provided with a limiter.

[0346] The limiter has a preset limit value, compares the output dataDmult fed thereinto, with the limit value, outputs the limit value ifthe limit value is smaller than the output data, and outputs the outputdata if the limit value is greater than the output data (the output isnamed as corrected image data Dlim in FIG. 12).

[0347] The limiter supplies the corrected image data Dlim, which iscompletely confined in the input range of the modulator, through theshift register and latch into the modulator.

[0348] (Gradation Converter)

[0349] Before the detailed description of the operation of the gradationconverter 200 in FIG. 12, a description will be given about a casewherein the image display apparatus is substantiated without correctionfor the influence of the voltage drop and without use of the gradationconverter 200.

[0350] Inventors confirmed the following phenomena in the image displayapparatus in the configuration without correction for the influence ofthe voltage drop and without the gradation converter 200.

[0351] Namely, the phenomena are as follows:

[0352] A. image data with the number of gradation levels being small(dark picture) provides more reddish display than in the case where asmall region is displayed in the number of gradation levels (pulse widthfor driving) of image data being 255 (in the 8-bit data width);

[0353] B. display becomes more reddish in the case of the display of theentire screen in the number of gradation levels of image data being 255than in the case where a small region is displayed in the number ofgradation levels (pulse width for driving) of image data being 255 (inthe 8-bit data width).

[0354] Inventors analyzed these phenomena and found the followingreason.

[0355] Namely, the red phosphor tended to make its emission efficiencysaturated at the high quantity of injected charge. For this reason,saturation became greater in the case where a small region was displayedin the number of gradation levels of image data being 255. Namely, theemission current is large because of the small voltage drop on thescanning wiring, and thus electrons impinge upon the phosphor over arelatively long period of time, so as to increase the quantity ofinjected charge and thus make the red phosphor saturated. For thisreason, when comparison is made on the basis of the reference in thecase where the small region is displayed in the number of gradationlevels of image data being 255, the quantity of charge injected into thered phosphor is smaller in the above A. and B. cases, so that saturationis less in the red phosphor. This resulted in relatively increasing theintensity of emission of red light, so that the display image becamereddish.

[0356]FIG. 19 is a diagram schematically showing the gradationcharacteristics without correction for the voltage drop and without thegradation converter.

[0357] In FIG. 19, the horizontal axis represents the pulse width fordriving of the modulation wiring, and the vertical axis normalizedluminance obtained by normalization with respect to luminances of therespective colors in the case where the small region is displayed in thenumber of gradation levels of the image data being 255 (in the case ofthe voltage drop on the scanning wiring being almost zero). In FIG. 19,a1gb indicates the gradation characteristic of green and blue in thecase where the voltage drop in the scanning wiring is almost zero, anda1r the gradation characteristic of red in the case where the voltagedrop in the scanning wiring is almost zero.

[0358] In FIG. 19, c1gb represents the gradation characteristic of greenand blue in the case where the maximum voltage drop occurs with all thedisplay devices on the scanning wiring being turned on, and c1r thegradation characteristic of red in the case where the maximum voltagedrop occurs with all the display elements on the scanning wiring beingturned on.

[0359] The gradation characteristics c1gb, c1r were normalized withrespect to the luminances in the case where the small region wasdisplayed in the number of gradation levels of image data being 255.FIG. 19 is illustrated on the assumption that the normalized luminanceof green and blue becomes ¼ when the driving pulse width is 255.

[0360] In FIG. 19, b1gb indicates the gradation characteristic of greenand blue in the case where the voltage drop occurs so as to present theintermediate luminance between the luminance of a1gb and the luminanceof c1gb, and b1r the gradation characteristic of red at the samevoltage. Similarly, they were normalized with respect to the luminancesin the case where the small region was displayed in the number ofgradation levels of image data being 255.

[0361] The characteristics of luminance and driving pulse width (valuesof corrected image data) shown in FIG. 19 vary depending upon voltagedrop amounts, and driving voltages of the electron-emitting devicesduring display of actual images vary depending upon images and positionsof devices. It was, therefore, difficult to realize a conversion capableof completely canceling the above characteristics.

[0362] Inventors conducted elaborate research and found the followingfeatures in driving with correction for the influence of the voltagedrop of the display panel using the surface conduction electron-emittingdevices.

[0363] (1) In the method according to the embodiment of the presentinvention with correction for the influence of the voltage drop, datawith adjusted pulse widths (corrected image data) is calculated forinput image data so as to be emitted charge amounts of products of theemission current amounts IE without any voltage drop and the pulsewidths determined by the image data, and the modulator drives thedisplay panel by the pulse widths.

[0364] (2) When the maximum of the corrected image data falls outsidethe input range of the modulator, the overflow process is carried out soas to multiply the corrected image data by the gain. The corrected imagedata is thus set in the input range of the modulator.

[0365] (3) The saturation characteristics of phosphors (in particular, ared phosphor) are characteristics almost determined by emitted chargeamounts in the range of pulse widths and emission current values of theelectron-emitting devices under the actual driving conditions of thedisplay panel.

[0366] Namely,

[0367] the feature (1) indicates that “with correction for the influenceof the voltage drop, charges enter the phosphors in the emitted chargeamounts of the products of the emission currents IE without any voltagedrop and the pulse widths determined by image data, independent of thevoltage drops actually occurring in the display panel and the pulsewidths for actual driving” (which means that emitted charge amountcorrection is implemented to make correction for variation of emittedcharge amounts so as to achieve the charge amounts corresponding to theimage data).

[0368] The feature (2) indicates that “with execution of the overflowprocess, charges enter the phosphors in the emitted charge amounts ofthe products of the emission currents IE without any voltage drop andthe pulse widths determined by the values of the image data times thegain.”

[0369] Furthermore, the feature (3) indicates that “the saturationcharacteristics of phosphors (in particular, red phosphor) can bedetermined by only the emitted charge amounts.”

[0370] Inventors invented the image display apparatus in theconfiguration with the gradation converter 200 as a result ofdeliberation on the above features (1), (2), and (3).

[0371] The gradation conversion characteristics of the gradationconverter 200 will be briefly described, prior to the description of theactual configuration of the gradation converter 200.

[0372] In FIG. 20, the horizontal axis represents the emitted chargeamounts from the surface conduction electron-emitting devices, and thevertical axis the luminances of the respective colors. For simplifyingthe description in FIG. 20, the emitted charge amounts on the horizontalaxis are presented as normalized quantities with respect to 1 set as acharge amount in the case where an emission current amount IE withoutany voltage drop is injected for only a time Δt equivalent to onegradation level of pulse width modulation. As a result of thenormalization, the maximum of emitted charge amount becomes 255. Namely,when a small area is displayed by the driving pulse width of themodulator of 255 (maximum) (i.e., when the voltage drop on the scanningwiring is almost zero), the emitted charge amount (maximum emittedcharge amount) is 255.

[0373] The vertical axis shows the luminances normalized with respect to1 as a luminance of each color where the emission current amount IEwithout any voltage drop is injected in the case of the pulse width ofthe 255 gradation level (255×Δt).

[0374] When the correction is made for the influence of the voltage dropin the embodiment of the present invention, the pulse width is adjustedso that a charge is injected into each phosphor in the emitted chargeamount of the product of the emission current amount IE without anyvoltage drop and the pulse width determined by the image data (thefeature (1)).

[0375] For this reason, in the case of the correction for the influenceof the voltage drop, the horizontal axis corresponds to 0 to 255 ofimage data.

[0376] In FIG. 20 qgb represents the gradation characteristic of greenand blue, and qr the gradation characteristic of red. FIG. 20 can beobtained by actual measurement with varying pulse widths or emissioncurrents (driving voltages), for example.

[0377] Since the emitted charge amounts are equivalent to the image datain the case without execution of the overflow process, it is understoodthat correction should be made so as to effect gradation conversion tocancel the characteristics of FIG. 20, on the image data. When thegradation converter 200 is provided with such conversion characteristicsas to cancel the gradation characteristics of FIG. 20, it becomesfeasible to overcome the aforementioned problem of reddish display.

[0378]FIG. 21 shows the actual gradation conversion characteristics forcanceling the characteristics of FIG. 20. FIG. 21 shows a case whereinput and output are 8-bit data. In FIG. 21, QGB represents acharacteristic curve to cancel the saturation characteristic of thegreen and blue phosphors (which is illustrated as a straight line on theassumption that they are not saturated in the present example), and QR acharacteristic curve to cancel the saturation characteristic of the redphosphor indicated by qr in FIG. 20.

[0379] Since the image data corresponds to the emitted chargeamounts(the feature (1)), as described previously, the gradationconversion of image data made it feasible to cancel the characteristicof the red phosphor having the saturation characteristic depending uponemitted charge amounts

[0380] Namely, the gradation conversion of image data means to convertthe image data as luminance requirements to emitted charge amountrequirements taking account of the emission characteristic of thephosphor.

[0381] It thus indicates correction for emitted charge amounts to makecorrection for variation of emitted charge amounts toward the emittedcharge amount requirements.

[0382] Next described is the case including the overflow process.According to the aforementioned feature (2), the charge enters eachphosphor in the emitted charge amount of the product of the emissioncurrent amount IE without any voltage drop and the pulse widthdetermined by the value of the image data times the gain (factor).

[0383] Namely, even if the input image data is the same, the emittedcharge amount with the overflow process is the gain times that withoutthe overflow process.

[0384] For detailed description, FIG. 22 shows the characteristics ofnormalized charge amount versus luminance. In FIG. 22, similar to FIG.20, emitted charge amounts on the horizontal axis indicate normalizedvalues with respect to 1 as a charge amount in the case where theemission current amount IE without any voltage drop is injected for atime Δt equivalent to one gradation level of pulse width modulation. Thevertical axis indicates normalized luminances with respect to 1 as aluminance of each color in the case where the emission current amount IEwithout any voltage drop is injected in the pulse width of the 255gradation level (255×Δt).

[0385] The characteristics qgb, qr in FIG. 22 are the same as theaforementioned characteristics in FIG. 20; qgb the gradationcharacteristic of green and blue and qr the gradation characteristic ofred. A square region indicated by GA in FIG. 22 is a region indicatingthe emitted charge amount-luminance with the gain of 1, and thenormalized charge amounts 0 to 255 on the horizontal axis correspond tothe image data 0 to 255 (equivalent to those in the aforementioned casewithout the overflow process).

[0386] When the gain is ½, the amount of charge injected into eachphosphor is equal to the charge amount of the image data times the gain(½), and thus the image data 0 to 255 corresponds to the normalizedcharge amounts 0 to 127. In FIG. 22 a square region indicated by GB is aregion indicating the emitted charge amount-luminance actually obtainedwhen the gain is ½.

[0387] Similarly, when the gain is ¼, the amount of charge injected intoeach phosphor is equal to the charge amount of the image data times thegain (¼), and thus the image data 0 to 255 corresponds to the normalizedcharge amounts 0 to 63. In FIG. 22 a square region indicated by GC is aregion indicating the emitted charge amount-luminance actually obtainedwhen the gain is ¼.

[0388] When the gain is G1, the amount of charge injected into eachphosphor is equal to the charge amount of the image data times the gain(G1), and thus the image data 0 to 255 corresponds to the normalizedcharge amounts 0 to (255×G1). In FIG. 22 a square region indicated by GGis a region indicating the emitted charge amount-luminance actuallyobtained when the gain is G1.

[0389] As described above, the emitted charge amounts actually achievedcorrespond to values of the product of the image data and the gain (anoperating point determined by the gain).

[0390] For this reason, the gradation conversion of image data asdescribed below makes it feasible to cancel the saturationcharacteristics of phosphors.

[0391] With the gain of 1, the normalized charge amounts 0 to 255correspond to the image data 0 to 255, and thus the saturationcharacteristic of the red phosphor can be canceled out by a γ correctiontable having the conversion characteristics shown in FIG. 23.

[0392] In FIG. 23, QGB indicates a characteristic curve to cancel thesaturation characteristic of the green and blue phosphors (which is astraight line on the assumption that no saturation occurs in the presentexample), and QR (×1) a characteristic curve to cancel the saturationcharacteristic of the red phosphor indicated by qr in FIG. 22.

[0393] Likewise, with the gain of ½, the normalized charge amounts 0 to127 correspond to the image data 0 to 255, and thus the saturationcharacteristic of the red phosphor can be canceled by a γ correctiontable having the conversion characteristics shown in FIG. 24.

[0394] In FIG. 24, QGB indicates a characteristic curve to cancel thesaturation characteristic of the green and blue phosphors (which is astraight line on the assumption that no saturation occurs in the presentexample), and QR (×½) a characteristic curve to cancel the saturationcharacteristic of the red phosphor indicated by the GB region of qr inFIG. 22.

[0395] When the reference is set in the non-saturated case like QGB, thegradation conversion of image data is to convert the image data (inputdata) 0 to 255 in the range of output data of 0 to 255 (the rangehereinafter will be described on the basis of the reference in thenon-saturated case like QGB).

[0396] The output data in the range of 0 to 255 is multiplied by thegain through the correction for the influence of the voltage drop, andthe normalized charge amounts of charges injected into the phosphorsthereafter fall in the range of 0 to 127.

[0397] The conversion to cancel the saturation characteristics of thephosphors at the operating point corresponding to the gain is carriedout as described above.

[0398] In other words, the conversion characteristics to cancel thesaturation characteristics of the phosphors can be obtained byconverting the image data (input data) 0 to 255 in the range of outputdata of 0 to 255, independent of the gain.

[0399] Similarly, with the gain of ¼, the normalized charge amounts 0 to63 correspond to the image data 0 to 255, and thus the saturationcharacteristic of the red phosphor can be canceled by a γ correctiontable having the conversion characteristics shown in FIG. 25.

[0400] In FIG. 25, QGB indicates a characteristic curve to cancel thesaturation characteristic of the green and blue phosphors (which is astraight line on the assumption that no saturation occurs in the presentexample), and QR (×¼) a characteristic curve to cancel the saturationcharacteristic of the red phosphor indicated by the GC region of qr inFIG. 22.

[0401] Similarly, with the gain of G1, the normalized charge amounts 0to (255×G1) correspond to the image data 0 to 255, and thus the imagedata is multiplied by the gain (G1) and is converted by the y correctiontable having the characteristics indicated by QGR and QR (×1) in FIG.23, thereby making correction for the influence of saturation of thephosphors. Furthermore, the output converted by the γ correction tableis multiplied by 1/gain (1/G1) to obtain the output data in the range of0 to 255 for the correction for the voltage drop.

[0402] In other words, the above-stated characteristics are equivalentto selection of the gradation conversion characteristics at theoperating point determined by the gain.

[0403] By the characteristics of the gradation converter 200 asdescribed above, it becomes feasible to cancel the aforementionedproblem of reddish display even in the case where the overflow processis carried out.

[0404] A practical configuration of the gradation converter 200 will bedescribed below.

[0405]FIG. 26 shows the configuration of the gradation converter 200. InFIG. 26 numerals 201 and 203 designate multipliers, 202 a γ correctiontable substantiated by a memory or the like, and 204 an inverse numbergenerator. This configuration realizes the aforementioned function. FIG.26 shows only the configuration corresponding to one color, forsimplification. It is a matter of course that the gradation converter200 is comprised of three sets of the same structure for red, green, andblue. In this case, the contents of the γ correction tables are preparedaccording to the saturation characteristics of the phosphors of therespective colors.

[0406] The input image data is multiplied by the gain (G1) at themultiplier 201. As described previously, the input image data ismultiplied by the gain to be converted into the emitted charge amount,and the γ correction table 202 effects the gradation conversion tocancel the saturation characteristic of the phosphor as normalized bythe maximum emitted charge amount (in the range of 1 to 255).

[0407] The γ correction table 202 implements the gradation conversion tocancel the saturation characteristic of the phosphor at the emittedcharge amount of charge actually emitted.

[0408] Since in this state the output of the γ correction table isgain-multiplied data, the multiplier 203 multiplies the output by 1/gain(1/G1) in order to recover data to be actually subjected to thecorrection for the voltage drop. The inverse number generator 204outputs an inverse number of the gain.

[0409] Since the gain is generally smaller than 1, it is necessary toset the bit count of the γ correction table 202 greater than the bitcount of the image data in order to maintain significant digits, becausethe image data times the gain is fed into the γ correction table 202.

[0410] The above configuration realized the aforementioned functionwhereby the aforementioned problem of reddish display was successfullyovercome by hardware.

[0411] Further, when there is a relation:

G=Kg×INMAX/MAX,

[0412] wherein Kg is a constant and meets:

Kg≦1.

[0413] Then,

1/G=MAX/(Kg×INMAX).

Kg×INMAX is a constant,

Kg′ is defined as a new constant:

Kg′=1/(Kg×INMAX).

[0414] Then,

1/G=Kg′×MAX.

[0415] That is, an inverter 204 can calculate an inverse of a gain bymultiplying a maximum value MAX of the correction image data by theconstant Kg′. In view of the above, the inventor constituted by ROM orthe like can be replaced by a multiplier, thereby reducing hardwarescale and parts.

[0416] Further, in case of calculating the second gain (Eq 21), also,similarly, there is a relation:

G 1=Kg 1×INMAX/AMAX,

[0417] wherein Kg1 is a constant and meets:

Kg1≦1.

[0418] Then,

1/G=AMAX/(Kg 1×INMAX).

Kg1×INMAX is a constant.

Kg1′ is defined as a new constant:

Kg 1′=1/(Kg×INMAX).

[0419] Then,

1/G=Kg 1′×AMAX.

[0420] That is, the inverter 204 can calculate an inverse of a gain bymultiplying AMAX derived by smoothing (averaging) in a frame direction amaximum value of the correction image data detected at a frame prior toa present frame by the constant Kg1′. In view of the above, the inverterconstituted by ROM or the like can be replaced by a multiplier, therebyreducing a hardware scale and parts.

[0421] Next, third gain calculating method is performed by calculatingthe gain according to the Eq 20 and averaging. In this case, by means ofthe calculation similar to the above second method, the inverter can bereplaced by the multiplier. While, it is necessary to perform theaveraging in both of the gain and the maximum value of the correctionimage data, separately. Thus, the hardware for performing the processingincreases. However, total hardware would be still smaller than thestructure using the inverter.

[0422] γ correction table 202 for eliminating the saturation of thephosphor may have following structure.

[0423] When the characteristics of γ correction table 202 is such that arequired luminance is Lr and a change quantity to be applied to thephosphor is qr, and the required luminance Lr and the charge quantity qrare both normalized as:

qr=fr(Lr),

[0424] wherein fr (Lr) is a characteristics stored in the γ correctiontable 202 for correcting the saturation of the phosphor.

[0425] And gr (Lr) is defined as a function:

gr(Lr)=Lr−fr(Lr).

[0426] That is, gr (Lr) is a function of a difference of thecharacteristics correlative to the luminance and the charge quantity.

[0427] In order to eliminate the saturation of the phosphor, it isnecessary to meet a relation:

qr=Lr−gr(Lr).

[0428] In the above described embodiments, the γ correction table 202may comprise a table of a characteristics gr (Lr) and a subtracter forsubtraction between outputs of tables of characteristics Lr to gr (Lr).In such case, the subtracter should be necessary to constitute hardwarestructure. However, there would be provided an advantage that, in casethat the table of the characteristics gr (Lr) is used for the memoriesof the same capacity, the gradation member can increase, and processingaccuracy would be improved.

[0429]FIG. 27 shows another embodiment of the gradation converter. FIG.27 also shows only a configuration for one color, for simplification. Ofcourse, the gradation converter 200 is constructed of three sets of thesame structure for red, green, and blue. In this case, the contents ofthe respective γ correction tables are prepared according to thesaturation characteristics of the phosphors of the respective colors.

[0430] In FIG. 27, 202a, 202 b, and 202 c designate γ correction tables,each of which stores a conversion table to cancel the saturationcharacteristic of the phosphor corresponding to the emitted chargeamounts of charge actually emitted when the gain is 1, ½, or ¼,respectively. In practice, the conversion tables are equivalent to theaforementioned characteristics shown in FIG. 23, FIG. 24, and FIG. 25.Numeral 205 denotes a linear interpolation unit, which is a unit ofaccepting input of the gain G1, and implementing linear interpolationfrom outputs of two tables on the both sides of the gain G1 out of the γcorrection tables 202 a, 202 b, and 202 c to obtain interpolation valuesfor the gain G1.

[0431] Since the characteristic to cancel the saturation characteristicof the phosphor, which is determined by the gain, varies monotonically,the above configuration permits the conversion characteristic for anygain G1 to be determined by the linear interpolation from thecharacteristics of the respective γ correction tables 202 a, 202 b, and202 c.

[0432] As the number of γ correction tables increases, the accuracybecomes more enhanced, while the cost of hardware rises, naturally. Withuse of three or more γ correction tables, it was feasible to preventdistinct degradation of display quality.

[0433] The above configuration also succeeded in realizing theaforementioned function whereby the aforementioned problem of reddishdisplay was successfully overcome by hardware.

[0434] Furthermore, it was described in the above description of thepresent embodiment that the gradation characteristics of the green andblue phosphors were high in linearity and had no saturationcharacteristic, but a luminance characteristics of the green and bluephosphors has saturation characteristic in relation to an electriccharge quantity even though it is much smaller than that of the redphosphor. In this case, it is also possible to obtain a normalizedgradation characteristic as described above for low saturation in eachcolor phosphor, prepare a table to cancel the characteristic, for eachcolor, and thereby make correction for the saturation characteristic ofthe phosphor of each color.

[0435] Further, the saturation characteristics of the phosphor variesaccording to an acceleration voltage (high power source voltage) betweenthe rear plate and the face plate and to a maximum charge quantityapplied to the phosphor. In case of driving a panel, since driving timesof respective electron emitting devices are defined, the maximum chargequantity to be applied to the phosphor is dependent on an emissioncurrent IE of the electron emitting device i.e., a voltage (Vs) of ascanning unit and a voltage (Vpwm) of a modulating unit. The saturationcharacteristics of the phosphor varies according to the voltage of thehigh power source, the voltage (Vs) of the scanning unit, and thevoltage (Vpwm) of the modulating unit. For an initial adjustment toeliminate performance characteristic variation of the display apparatus,and for adjusting by user, by adjusting the voltage of the high powersource, the voltage (Vs) of the scanning unit and the voltage (Vpwm) ofthe modulating unit, it would be desirable to connect to the γcorrection table to cancel the saturation characteristics of thephosphor for the corresponding voltage.

[0436] (Shift Register and Latch)

[0437] The corrected image data Dlim being the output of the limiter isconverted from the serial data format into the parallel image data ID1to IDN for the respective modulation wires by the serial/parallelconversion at the shift register 5, and the parallel image data isoutputted to the latch. The latch latches the data from the shiftregister in accordance with a timing signal Dataload immediately beforea start of one horizontal period. The output from the latch 6 issupplied as parallel image data D1 to DN into the modulator.

[0438] In the present embodiment the image data ID1-IDN, D1-DN is 8-bitimage data. These operations are activated based on the timing controlsignals TSFT and Dataload from the timing generator 4 (FIG. 12).

[0439] (Details of Modulator)

[0440] The parallel image data D1-DN being the output of the latch 6 issupplied into the modulator 8.

[0441] The modulator is, as shown in FIG. 28A, a pulse width modulationcircuit (PWM circuit) comprised of a PWM counter, and comparators andswitches (FETs in the same figure) for the respective modulation wires.

[0442] The relation between the image data D1-DN and the output pulsewidths of the modulator is the linear relation as shown in FIG. 28B.

[0443]FIG. 28C shows three examples of output waveforms from themodulator.

[0444] In FIG. 28C the top waveform is one for the input data of 0 intothe modulator, the middle waveform one for the input data of 128 intothe modulator, and the bottom waveform one for the input data of 255into the modulator.

[0445] In the present example the bit count of the input data D1-DN intothe modulator is 8 bits.

[0446] In the above description there were portions describing that themodulation signal of the pulse width equivalent to one horizontalscanning period was outputted for the input data of 255 into themodulator, but, precisely speaking, there are idle periods provided astimewise margins before the rise of the pulse and after the fall of thepulse though they are very short durations, as shown in FIG. 28C.

[0447]FIG. 29 is a timing chart showing the operation of the modulatorin the embodiment of the present invention.

[0448] In the same figure, Hsync designates a horizontal synchronizingsignal, Dataload a load signal to the latch 6, D1-DN the aforementionedinput signals into the columns 1-N of the aforementioned modulator,Pwmstart a synchronizing clear signal of the PWM counter, and Pwmclk aclock of the PWM counter. XD1-XDN denote outputs of the first to Nthcolumns of the modulator.

[0449] As shown in FIG. 29, with a start of one horizontal scanningperiod, the latch 6 starts latching the image data and transferring thedata to the modulator.

[0450] The PWM counter starts counting on the basis of Pwmstart andPwmclk, as shown in the same figure, stops counting at the count valueof 255, and retains the count of 255.

[0451] The comparator for each column compares the count of the PWMcounter with the image data of each column, outputs High during periodsin which the value of the PWM counter is not less than the image data,and outputs Low during the other periods.

[0452] The output of each comparator is coupled to gates of switches foreach column, and in each Low period of output of the comparator theupper switch (Vpwm side) in the same figure is ON and the lower switch(GND side) OFF, whereby the modulation wire is connected to the voltageVpwm.

[0453] During each High period of output of the comparator contrary, theupper switch in the same figure is OFF and the lower switch ON, wherebythe modulation wire is connected to the GND potential.

[0454] When each of the units operates as described above, the pulsewidth modulation signals from the modulator become signals of waveformswith a synchronized rise of pulses, as indicated by XD1, XD2, and XDN inFIG. 29.

[0455] (Correction Data Calculator)

[0456] The correction data calculator is a circuit of calculating thecorrection data for the voltage drop by the aforementioned correctiondata calculation method. The correction data calculator is comprised oftwo blocks of a discrete correction data calculating unit and acorrection data interpolating unit as shown in FIG. 30.

[0457] The discrete correction data calculating unit calculates thevoltage drop amounts from the input video signal and discretelycalculates the correction data from the voltage drop amounts. Thiscalculating unit discretely calculates the correction data by adoptingthe aforementioned concept of the degenerate model, in order to decreasethe calculation amount and hardware scale.

[0458] The correction data interpolating unit performs the interpolationoperation from the discretely calculated correction data to calculatethe correction data CD matching the sizes of image data and thehorizontal display positions x.

[0459] (Discrete Correction Data Calculating Unit)

[0460]FIGS. 31A and 31B show the discrete correction data calculatingunit for calculating the discrete correction data in the embodiment ofthe present invention.

[0461] The discrete correction data calculating unit is, as describedbelow, a unit having a function of grouping the image data into blocks,calculating statistics (numbers of lighting devices) of the respectiveblocks, and calculating time changes of voltage drop amounts at thepositions of the respective nodes from the statistics, a function ofconverting the voltage drop amounts at the respective times intoemission luminance amounts, and a function of integrating the emissionluminance amounts over the time to obtain the total emission luminanceamounts; and calculating the correction data for the reference values ofimage data at the discrete reference points from the total emissionluminance amounts.

[0462] In FIGS. 31A and 31B, 100 a-100 d designate lighting devicecounting units; 101 a-101 d register groups for storing the numbers oflighting devices at respective times, for the respective blocks; 102 aCPU; 103 a table memory for storing the parameters aij in Eqs 2 and 3;104 a temporary register for temporarily storing the calculation result;105 a program memory which stores programs of CPU; 111 a table memorywhich stores the conversion data for conversion of voltage drop amountsinto emission current amounts; 106 a register group for storing theresult of calculation of the aforementioned discrete correction data.

[0463] Each of the lighting device counting units 100 a-100 d iscomprised of comparators and adders as shown in FIG. 31B. The videosignals Ra, Ga, and Ba are fed into the respective comparators 107 a-107c, to be compared with the value of Cval one by one.

[0464] Cval is equivalent to the image data reference value set for theimage data as described previously.

[0465] Each comparator 107 a-c compares the image data with Cval andoutputs High if the image data is larger, or outputs Low if it issmaller.

[0466] The adders 108 and 109 add up the outputs from the comparators,and the adder 110 further adds up the data in every block. The result ofthe addition in each block is stored as the number of lighting devicesin each block in the register group 101 a-c.

[0467] The lighting device counting units 100 a-d accept theirrespective inputs of 0, 64, 128, and 192, respectively, as thecomparison value Cval of the comparators.

[0468] As a result, the lighting device counting unit 100 a counts thenumber of image data greater than 0 out of the image data and stores thesums of the respective blocks in the register 101 a.

[0469] Likewise, the lighting device counting unit 100 b counts thenumber of image data greater than 64 out of the image data and storesthe sums of the respective blocks in the register 101 b.

[0470] Similarly, the lighting device counting unit 100 c counts thenumber of image data greater than 128 out of the image data and storesthe sums of the respective blocks in the register 101 c.

[0471] Similarly, the lighting device counting unit 100 d counts thenumber of image data greater than 192 out of the image data and storesthe sums of the respective blocks in the register 101 d.

[0472] After completion of counting the lighting devices in therespective blocks and at the respective times, the CPU reads theparameter table aij stored in the table memory 103, as needed,calculates the voltage drop amounts according to Eqs 2 to 5, and storesthe calculation results in the temporary register 104.

[0473] In the present example the CPU is provided with thesum-of-products operation function for smoothly performing thecalculation of Eq 2.

[0474] The means for implementing the operation according to Eq 2 doesnot always have to be the means performing the sum-of-products operationin the CPU, but it may be substantiated, for example, by storing thecalculation results in a memory.

[0475] Namely, it can be contemplated that a memory is configured toaccept input of the numbers of lighting devices in the respective blocksand store voltage drop amounts at the respective node positions for allpossible input patterns.

[0476] After completion of the calculation of voltage drop amounts, theCPU reads the voltage drop amounts at the respective times and in therespective blocks out of the temporary register 104, makes reference tothe table memory 2 (111) to convert the voltage drop amounts intoemission current amounts, and calculates the discrete correction dataaccording to Eqs 6 to 9.

[0477] The discrete correction data thus calculated is stored in theregister group 106.

[0478] (Correction Data Interpolating Unit)

[0479] The correction data interpolating unit is a unit for calculatingthe correction data matching the display positions (horizontalpositions) of the image data and the sizes of image data. This unitperforms the interpolation from the discretely calculated correctiondata to obtain the correction data according to the display positions(horizontal positions) of the image data and the sizes of the imagedata.

[0480]FIG. 32 is a diagram for explaining the correction datainterpolating unit.

[0481] In FIG. 32 numeral 123 designates a decoder for determining nodenumbers n and n+1 of discrete correction data used for theinterpolation, from the display position (horizontal position) x ofimage data, and numeral 124 a decoder for determining k and k+1 in Eq 17to Eq 19 from the size of the image data.

[0482] Selectors 125 to 128 are selectors for selecting discretecorrection data and supplying it to linear approximation units.

[0483] Numerals 121 to 123 denote linear approximation units foreffecting the linear approximations of Eq 17 to Eq 19, respectively.

[0484]FIG. 33 shows a configuration example of the linear approximationunit 121. As seen from the operators in Eq 17 to Eq 19, the linearapproximation unit can be generally comprised of subtracters,accumulators, an adder, and a divider, for example.

[0485] However, the linear approximation unit is preferably configuredso that the number of column wires between nodes for calculation of thediscrete correction data and intervals of image data reference valuesfor calculation of the discrete correction data (i.e., time intervalsfor calculation of voltage drop) are powers of 2, because it can providethe advantage that the hardware can be constructed in very simplestructure. When those are set in powers of 2, (Xn+1−Xn) in the dividershown in FIG. 33 becomes a value of a power of 2, which can beimplemented simply by a bit shift.

[0486] If the value of (Xn+1−Xn) is always a constant value and valueexpressed by the power of 2, the output of the divider can be obtainedby shifting the addition result of the adder by the degree equivalent tothe power, and, therefore, the divider does not have to be provided.

[0487] In the other portions, when the intervals of the nodes forcalculation of discrete correction data and the intervals of the imagedata are powers of 2, it becomes feasible to readily fabricate thedecoders 123, 124, for example, and to replace the operations in thesubtracters in FIG. 33 with easy bit operations, thus providing manymerits.

[0488] (Operation Timing of Each Unit)

[0489]FIGS. 34A to 34C present a timing chart of operation timing ofeach unit.

[0490] In FIGS. 34A to 34C, Hsync denotes a horizontal synchronizingsignal; DotCLK a clock created from the horizontal synchronizing signalHsync by the PLL circuit in the timing generator; R, G, and B digitalimage data from the input switching circuit; Data image data after thedata sequence conversion; Dlim the output of the limiter, which is thecorrected image data after the correction for the voltage drop; TSFT ashift clock for transferring the corrected image data Dlim to the shiftregister 5; Dataload a load pulse signal for latching the data in thelatch 6; Pwmstart a start signal of the aforementioned pulse widthmodulation; the modulation signal XD1 an example of the pulse widthmodulation signal supplied to the modulation wiring 1.

[0491] With a start of one horizontal period, the digital image data RGBis transferred from the selector 23. Let R_I, G_I, and B_I be the inputimage data during the horizontal scanning period I in FIGS. 34A to 34C.Then those image data items are stored during one horizontal period inthe data sequence converter 9, and are outputted as digital image dataData_I in alignment with the pixel array of the display panel during thehorizontal scanning period I+1.

[0492] The image data items R_I, G_I, and B_I are fed into thecorrection data calculator during the horizontal scanning period I. Thiscalculator counts the aforementioned lighting devices and obtains thevoltage drop amounts at the end of the counting.

[0493] Subsequent to the calculation of voltage drop amounts, thediscrete correction data is calculated and the calculation result isstored in the register.

[0494] When the time moves into the scanning period I+1, the datasequence converter outputs the image data Data_I in the precedinghorizontal scanning period, and in synchronism therewith, the correctiondata interpolation unit performs the interpolation from the discretecorrection data to calculate the correction data. The correction dataafter the interpolation is supplied to the adder 12.

[0495] The adder 12 sequentially adds the correction data CD to theimage data Data and transfers the corrected image data Dlim after thecorrection to the shift register. The shift register stores thecorrected image data Dlim of one horizontal period according to Tsft andeffects serial-parallel conversion thereon to output parallel image dataID1-IDN to the latch 6. The latch 6 latches the parallel image dataID1-IDN from the shift register according to a rise of Dataload, andtransfers the latched image data D1-DN to the pulse width modulator 8.

[0496] The pulse width modulator 8 outputs the pulse width modulationsignals of the pulse widths according to the latched image data. In theimage display apparatus of the present embodiment, as a result, thepulse width modulation signals outputted from the modulator aredisplayed with a delay of two horizontal scanning periods behind theinput image data.

[0497] Display of images was actually carried out using the imagedisplay apparatus as described above, and it was verified that it wasfeasible to make the correction for the voltage drop amounts on thescanning wiring, which was the problem heretofore, to make improvementin the degradation of display images caused thereby, and to providedisplay of remarkably excellent images.

[0498] The adoption of the several approximations presented theexceptional effects of facilitating the proper calculation of correctionamounts of image data for the correction for the voltage drop,implementing it by very simple hardware, and so on.

[0499] (Second Embodiment)

[0500] The corrected image data Dout is the result of the addition ofimage data Data and correction data CD.

[0501] Unless the result of this addition falls within the input rangeof the modulator, the correction can induce overflow, which raised theconcern that another sense of strangeness appeared in the display image.

[0502] In order to solve this problem, the first embodiment wasconfigured to prevent the overflow by the method of detecting themaximum of corrected image data, calculating the gain so that themaximum became corresponding to the maximum of the input range of themodulator, and multiplying the corrected image data by the gain.

[0503] As compared therewith, the present embodiment is similar in thedetection of the maximum of the corrected image data, but is differentin that the size of the image data before execution of the correction islimited so that the aforementioned maximum becomes corresponding to themaximum of the input range of the modulator.

[0504] Namely, in order to prevent the overflow, the input image data ispreliminarily multiplied by a gain to narrow the amplitude rangethereof, thereby preventing the overflow.

[0505] The overflow process of the present embodiment will be describedhereinafter with reference to FIG. 35.

[0506] In FIGS. 35, 22R, 22G, and 22B denote multipliers; 9 a datasequence converter; 5 a shift register for one line of image data; 6 alatch for one line of image data; 8 a pulse width modulator foroutputting modulation signals into the modulation wires of the displaypanel; 12 an adder; 14 a correction data calculator; 20 a maximum valuedetector (unit) for detecting the maximum of corrected image data Doutin a frame; 21 a gain calculator; 200 a gradation converter. Thegradation converter 200 will be described later, and the followingdescription will be given on the assumption that the modulationconverter 200 is absent.

[0507] R, G, and B denote parallel RGB input video data; Ra, Ga, and Baparallel RGB video data after the inverse γ conversion process; Rx, Gx,and Bx image data resulting from the multiplication by GAIN G2 at themultipliers; GAIN G2 a gain calculated by the gain calculator; Dataimage data after the parallel-serial conversion at the data sequenceconverter; CD correction data calculated by the correction datacalculator; Dout image data corrected by the addition of correction dataand image data at the adder (corrected image data); Dlim corrected imagedata obtained by limiting Dout below the upper limit of the input rangeof the modulator, by the limiter.

[0508] (Multipliers 22R, 22G, 22B)

[0509] The multipliers 22R, 22G, and 22B are units for multiplying theimage data Ra, Ga, Ba after the inverse γ conversion by GAIN G2.

[0510] More specifically, each of the multipliers multiplies the imagedata by GAIN G2 according to the gain determined by the gain calculatorand outputs the image data Rx, Gx, Bx after the multiplication.

[0511] GAIN G2 is a value that is calculate by the gain calculator andthat is determined so that the corrected image data Dout, which is theresult of the addition of the image data Data and the correction data atthe adder described hereinafter, falls within the input range of themodulator.

[0512] (Maximum Detector 20)

[0513] The maximum detector 20 will be described below.

[0514] The maximum detector in the embodiment of the present inventionis coupled to each of the units, as shown in FIG. 35.

[0515] The maximum detector is a unit of detecting a maximum value outof the corrected image data Dout of one frame.

[0516] This detector is a circuit that can be readily constructed of acomparator and a register, for example. This detector is a circuit ofcomparing the size of corrected image data Dout sequentially transferredthereto, with the value stored in the register and updating the value ofthe register with the data value if the corrected image data Dout isgreater than the register value.

[0517] The value of the register is cleared to 0 at the head of a frame,and then a maximum MAX of the corrected image data in that frame isstored in the register at the time of the end of the frame.

[0518] The maximum MAX of the corrected image data detected in this wayis transferred to the gain calculator.

[0519] (Gain Calculator)

[0520] The gain calculator is a unit of calculating the gain so that thecorrected image data Dout falls within the input range of the modulator,with reference to the detected value MAX of the maximum detector. In thepresent embodiment, the gain calculator also calculates the gain foradjustment of the amplitude of the corrected image data on the basis ofthe adaptive gain method.

[0521] Alternatively, the gain may be calculated by the fixed gainmethod, from the viewpoint of preventing the overflow of the correctedimage data in the configuration of the present embodiment (FIG. 35).

[0522] The gain determining method can be a method of determining thegain so as to satisfy the following condition:

GAIN G 2≦(INMAX/MAX)×GB  (Eq 22),

[0523] where MAX is the maximum of corrected image data Dout in oneframe, INMAX the maximum of the input range of the modulator, and GBGAIN G2 calculated for an immediately preceding frame by the gaincalculator.

[0524] This gain calculator updates the gain in a vertical trace periodand changes the value of the gain every frame.

[0525] In the configuration of the image display apparatus according tothe embodiment of the present invention, the gain by which the correctedimage data of the current frame is multiplied is calculated using themaximum of the corrected image data of the preceding frame.

[0526] Namely, the apparatus is configured to prevent the overflow bymaking use of the correlation of corrected image data (image data)between frames.

[0527] Strictly speaking, overflow can occur because of the differenceof corrected image data between frames accordingly.

[0528] In order to solve this problem, the circuit was designed with alimiter located at the output of the multiplier for multiplying thecorrected image data by the gain so that the output of the multiplieralways fell into the input range of the modulator, and it presentedbetter result.

[0529] Inventors confirmed that the gain could be calculated by anothermethod as described below, in addition to the above gain determiningmethod.

[0530] Namely, the gain for the corrected image data of the currentframe can be determined so as to satisfy the following condition:

GAIN G 2≦(INMAX/AMAX)×GB  (Eq 23),

[0531] where AMAX is an average obtained by averaging maximums ofcorrected image data detected in frames preceding to the current frame.

[0532] In the above condition GB denotes GAIN G2 calculated for theimmediately preceding frame by the gain calculator.

[0533] Another method can be a method of calculating GAIN G2 for eachframe according to Eq 22 and averaging gains of respective frames toobtain the current gain.

[0534] Inventors confirmed that any of these three methods was suitablyapplicable in terms of the prevention of overflow and concluded that thegain was preferably calculated by the method of Eq 23 in considerationof occurrence of flicker as described in the first embodiment.

[0535] Inventors conducted research on the number of frames to be usedfor the averaging of maximums of corrected image data in the gaincalculation method of Eq 23 and found a preferred configuration ofaveraging the maximums of corrected image data in the range of 16 to 64frames preceding to the current frame, with better result.

[0536] It is needless to mention that the present method is configured,more preferably as shown in FIG. 35, so as to prevent the overflowcompletely by the limiter for limiting the output of the adder.

[0537] The gain calculation method may also be modified based ondetection of the scene change, as in the first embodiment.

[0538] (Gradation Converter)

[0539] In the second embodiment, much the same phenomena as in the firstembodiment were recognized in the absence of the gradation converter200.

[0540] The second embodiment is different only in the place for themultiplication by the gain in the overflow process, and is thus providedwith the gradation converter 200 in the configuration similar to that inthe first embodiment. The characteristics and configuration of thegradation converter are the characteristics of FIG. 22, FIG. 23, FIG.24, and FIG. 25 and the configuration of FIG. 26 or FIG. 27, as in thefirst embodiment. This configuration successfully canceled the influenceof saturation of phosphors and overcame the aforementioned problem ofreddish display.

[0541] When the configuration of the gradation connecter 200 is oneshown in FIG. 26, as shown in the first embodiment, a table of acharacteristics of gr (Lr) being a function of a difference betweencharacteristics correlative to a luminance and a charge quantity, and asubtracter for subtracting between outputs from tables of thecharacteristics Lr and gr (Lr) may be used.

[0542] And, when the configuration of the gradation converter 200 in thesecond embodiment is the configuration shown in FIG. 26, the multipliers22R, 22G, 22B in FIG. 35 and the multiplier 203 and the inverse numbergenerator 204 in FIG. 26 are omissible. The reason is that the inputdata into the multiplier 203 is multiplied by 1/gain in the multiplier203 and the output therefrom is further multiplied by the gain at eachmultiplier 22R, 22G, 22B, so that the input data into the multiplier 203is equal to the output data from the multipliers 22R, 22G, 22B.

[0543] The configuration in this arrangement is presented in FIG. 37 andFIG. 38. Since the structure and operation are the same, the descriptionthereof is omitted herein.

[0544] Furthermore, just as in the case of the first embodiment, it wasalso described in the second embodiment that the gradationcharacteristics of the green and blue phosphors were high in linearityand had no saturation characteristic, but in practice a luminancecharacteristics of the green and blue phosphors has a saturationcharacteristic in relation to an electric charge quantity, even thoughit is much smaller than that of the red phosphor. In this case, it isalso possible to employ a method of determining the aforementionednormalized gradation characteristic for low saturation in each color,preparing a table to cancel the characteristic, for each color, andthereby making correction for the saturation characteristic of thephosphor of each color.

[0545] It is also possible to downsize the hardware scale in such a waythat the characteristics of the γ tables 202 a, 202 b, 202 c in FIG. 27are determined in consideration of the characteristics of the inversethe γ processor 17 and the inverse γ processor 17 is excluded.

[0546] As described in the first embodiment, the saturationcharacteristics of the phosphor varies according to on accelerationvoltage (a voltage of the high power source) between the face plate andthe near-plate and to a maximum charge quantity applied to the phosphor.In driving the panel, since the driving times of respective electronemitting devices are determined, a maximum charge quantity to be appliedto the phosphor is dependent on an emission current IE of the electronemitting device, i.e., a voltage (Vs) of the scanning unit and a voltage(Vpwm) of the modulation unit. The saturation characteristics of thephosphor varies according to the voltage of the high power source, thevoltage (Vs) of the scanning unit and the voltage (Vpwm) of themodulating unit. For an initial adjustment for eliminating performancecharacteristic variations of the display apparatus, and for user'sadjustment, in case of varying the voltage of the high power source, thevoltage (Vs) of the scanning unit and the voltage (Vpwm) of themodulating unit, it is desirable to convert into γ correction tableeliminating the saturation characteristic variations of the phosphorcorresponding voltage.

[0547] Furthermore, in the case of the image display apparatus accordingto the embodiment of the present invention, with entry of nonzero,uniform image data common to all the colors, the apparatus is drivenwith the process of canceling the influence of the voltage drop so thata pulse width of a pulse from the modulator close to the outputterminals of the scanning circuit becomes shorter than a pulse width ofa pulse from the modulator far from the output terminals of the scanningcircuit.

[0548] Furthermore, as a result of canceling the saturationcharacteristics of phosphors dependent upon the emitted charge amountsof the electron-emitting devices, the driving is implemented withoutdeviation of luminance balance among displayed colors, i.e., at almosteven color temperature of white color, for any image data uniform andcommon to all the colors.

[0549] The embodiment of the present invention presented the example ofmaking the correction to cancel the saturation characteristics of thephosphors, and it is noted that the same configuration as in theembodiment of the present invention can also make correction for changeof gradation characteristics due to the electron emission amountsbecause of the influence of degradation of the driving voltage waveformsfor the electron-emitting devices (waveform rounding) or the like.

[0550] As described above, the image display apparatus of the presentinvention succeeded in making proper improvement in the degradation ofdisplay image due to the voltage drop on the scanning wiring, which wasthe problem heretofore.

[0551] The adoption of the several approximations provided theremarkably excellent effects of facilitating the proper calculation ofcorrected image data with correction for the influence of the voltagedrop, implementing it by very simple hardware, and so on.

[0552] Furthermore, the image display apparatus of the present inventionwas provided with the overflow processing circuit for preventing theoverflow of the image data after the correction from the input range ofthe modulator, whereby the overflow was prevented by the gain.

[0553] Since the gradation converter for changing the gradationconversion characteristics according to the gain was provided in thestage preceding to the configuration of making the correction for theinfluence of the voltage drop, it was feasible to successfully cancelthe saturation characteristics of the phosphors and thereby display theimages with high quality.

What is claimed is:
 1. An image display apparatus comprising: aplurality of image forming devices connected to a plurality of row wiresand column wires respectively and arranged in a matrix pattern; scanningmeans connected to said row wires; modulating means connected to saidcolumn wires; gradation converting means for converting a gradationcharacteristic of input image data; corrected image data calculatingmeans for calculating corrected image data, which is image data aftercorrection for influence of a voltage drop caused by a resistance ofsaid row wires and scanning means, for an output of the gradationconverting means; the modulating means outputting modulation signals tosaid column wires, with entry of the corrected image data, wherein saidgradation conversion characteristic is a characteristic of makingcorrection for a light emission characteristic of the image formingdevices in an absent state of the voltage drop.
 2. An image displayapparatus comprising: a plurality of image forming devices connected toa plurality of row wires and column wires respectively and arranged in amatrix pattern; scanning means connected to said row wires; modulatingmeans connected to said column wires; gradation converting means forconverting a gradation characteristic of input image data; correctedimage data calculating means for calculating corrected image data, whichis image data after correction for influence of a voltage drop caused bya resistance of said row wires and scanning means, for an output of thegradation converting means; and amplitude adjusting means having afunction of multiplying data by a factor for adjustment of the amplitudeof the corrected image data so that the amplitude of the corrected imagedata matches an input range of the modulating means, wherein saidgradation converting means has a gradation conversion characteristiccorresponding to said factor, and wherein said modulating means outputsmodulation signals to said column wires, with entry of the correctedimage data amplitude-adjusted by said amplitude adjusting means.
 3. Theimage display apparatus according to claim 2, wherein said image formingdevices are electron-emitting devices and wherein electrons emitted fromthe electron-emitting devices impinge upon a phosphor to induce lightemission thereof, and wherein said gradation converting means has afunction of changing the gradation conversion characteristic accordingto said factor so as to cancel a saturation characteristic of thephosphor at an operating point defined by said factor.
 4. The imagedisplay apparatus according to claim 3, wherein said gradationconverting means is comprised of a multiplier of multiplying image databy said factor, and a γ correction table of canceling the saturationcharacteristic of the phosphor in the absent state of the voltage drop,and is configured to feed an output of said multiplier into a γcorrection table of canceling a gradation characteristic of luminance inthe absent state of said voltage drop.
 5. The image display apparatusaccording to claim 3, wherein said gradation converting means comprisesa γ correction table of canceling the saturation characteristic of thephosphor in the absent state of the voltage drop and a γ correctiontable of canceling a gradation characteristic of luminance in a rangedetermined by said factor and is configured to perform interpolationbetween outputs of respective γ correction tables determined by factorsand output the result of the interpolation.
 6. The image displayapparatus according to claim 3, wherein the characteristic of saidgradation converting means is a characteristic of canceling greatersaturation of a phosphor when the factor is large than when the factoris small.
 7. The image display apparatus according to claim 2, whereinsaid amplitude adjusting means adjusts the amplitude of the correctedimage data outputted from the corrected image data calculating means, bymultiplying input image data before the correction, being an input intothe corrected image data calculating means, by a factor for adjustingthe amplitude thereof.
 8. The image display apparatus according to claim2, wherein said amplitude adjusting means detects a maximum of outputsof said corrected image data calculating means in each frame andadaptively calculates said factor so that the maximum matches an upperlimit of the input range of the modulating means.
 9. The image displayapparatus according to claim 2, wherein said factor is a factorpreliminarily determined so that with entry of maximum input image dataan output of said corrected image data calculating means does notoverflow the input range of said modulating means.
 10. The image displayapparatus according to claim 2, wherein said corrected image datacalculating means comprises: means for estimating a spatial distributionand a temporal change of voltage drop amounts to be caused on the rowwires during one horizontal scanning period, corresponding to inputimage data; and means for calculating corrected image data withcorrection for said input image data, from the voltage drop amountscalculated.
 11. The image display apparatus according to claim 2,wherein said corrected image data calculating means comprises: means fordiscretely estimating spatial distributions and temporal changes ofvoltage drop amounts to be caused on the row wires during one horizontalscanning period, corresponding to input image data; and means forcalculating corrected image data with correction for said input imagedata, from the voltage drop amounts calculated.
 12. The image displayapparatus according to claim 2, wherein said corrected image datacalculating means comprises: means for discretely estimating voltagedrop amounts to be caused on the row wires during one horizontalscanning period, in a spatial direction and in a temporal direction,corresponding to input image data; discrete corrected image datacalculating means for discretely calculating corrected image data forimage data corresponding to times of the calculation of said voltagedrop amounts at spatial positions of the calculation of said voltagedrop amounts, from said voltage drop amounts; and corrected image datainterpolating means for performing interpolation between outputs of thediscrete corrected image data calculating means to calculate correctedimage data matching sizes and horizontal display positions of the inputimage data.
 13. The image display apparatus according to claim 2,wherein said modulating means is pulse width modulation means forimplementing modulation by varying a pulse width of a voltage pulsewaveform applied to each column wire, according to an input into themodulating means.
 14. The image display apparatus according to claim 2,wherein said gradation converting means has a function of convertinginput image data into an emitted charge amount requirement to cancel asaturation characteristic of a phosphor, and outputting the emittedcharge amount requirement, and wherein said corrected image datacalculating means has a function of making correction for variation ofan emitted charge amount due to influence of said voltage drop, for theemitted charge amount requirement being an output of said gradationconverting means.
 15. The image display apparatus according to claim 2,wherein the corrected image data calculated by said corrected image datacalculating means is adjusted so that said emitted charge amountrequirement becomes an emitted charge amount in the absent state of thevoltage drop to be caused on said row wire.
 16. An image displayapparatus comprising: a plurality of electron-emitting devices connectedto a plurality of row wires and column wires respectively and arrangedin a matrix pattern; scanning means connected to said row wires;modulating means connected to said column wires; gradation convertingmeans for performing gradation conversion of input image data; correctedimage data calculating means for calculating corrected image data, whichis image data after correction for influence of a voltage drop caused bya resistance of said row wires and scanning means, for an output of thegradation converting means; and amplitude adjusting means having afunction of multiplying data by a factor for adjustment of the amplitudeof the corrected image data so that the amplitude of the corrected imagedata matches an input range of the modulating means, in which saidgradation converting means has a gradation conversion characteristiccorresponding to said factor, and in which said modulating means outputsmodulation signals to said column wires, with entry of the correctedimage data amplitude-adjusted; wherein with entry of nonzero, uniformimage data common to all colors, a pulse width of an output pulse fromthe modulating means close to an output terminal of said scanning meansbecomes shorter than a pulse width of an output pulse from themodulating means far from the output terminal of the scanning means, andsaturation characteristics of phosphors dependent upon emitted chargeamounts of the electron-emitting devices are further canceled, so as toimplement such driving that any image data uniform and common to all thecolors is displayed at almost equal color temperature of white color,independent of emission luminance.
 17. An image display method by animage display apparatus comprising a plurality of electron-emittingdevices connected to a plurality of row wires and column wires one eachrespectively and arranged in a matrix pattern, scanning means connectedto said row wires, modulating means connected to said column wires, andphosphors opposed to said electron-emitting devices, said image displaymethod comprising: a step of calculating emitted charge amountrequirements with correction for light emission characteristics of thephosphors against emitted charge amounts, according to image data asluminance requirements; and a step of calculating corrected image datawith correction for variation of the emitted charge amounts due toinfluence of a voltage drop caused by a resistance of said row wires andscanning means, according to said emitted charge amount requirementscalculated, wherein said modulating means applies pulse waveformsaccording to the corrected image data thus calculated, to said columnwires.
 18. An image display method by an image display apparatuscomprising a plurality of electron-emitting devices connected to aplurality of row wires and column wires one each respectively andarranged in a matrix pattern, scanning means connected to said rowwires, and modulating means connected to said column wires, said imagedisplay method comprising: a step of performing gradation conversion ofcanceling a light emission characteristic of the electron-emittingdevices in an absent state of a voltage drop caused by a resistance ofsaid row wires and scanning means, for input image data; and a step ofmaking correction for influence of the voltage drop caused by theresistance of said row wires and scanning means, for an output in thestep of performing the gradation conversion of canceling said lightemission characteristic, wherein said modulating means applies pulsewaveforms to said column wires according to an output in the step ofmaking the correction for the influence of the voltage drop.
 19. Animage display method by an image display apparatus comprising aplurality of electron-emitting devices connected to a plurality of rowwires and column wires one each respectively and arranged in a matrixpattern, scanning means connected to said row wires, and modulatingmeans connected to said column wires, said image display methodcomprising: a step of converting a gradation characteristic of inputimage data; a step of making correction for influence of a voltage dropcaused by a resistance of said row wires and scanning means, for anoutput in the step of converting said gradation characteristic, in whichsaid modulating means applies pulse waveforms to said column wiresaccording to an output in the step of making the correction for theinfluence of the voltage drop, wherein said step of making thecorrection for the influence of the voltage drop further comprises astep of adjusting the amplitude so that the output in the step of makingthe correction for the influence of the voltage drop falls within aninput range of the modulating means, and wherein said step of convertingthe gradation characteristic is to select a portion of a characteristicof canceling a light emission characteristic of the electron-emittingdevices in an absent state of the voltage drop caused by the resistanceof said row wires and scanning means, according to an output in the stepof adjusting the amplitude so that the output falls in the input rangeof said modulating means.